There's a couple of things to note with the code you just posted:

(1) If only 1 file is selected, element 0 contains the full path and file
(2) There's no need to trap an error if the user doesn't select a file. The UBound is set to -1 in that case.

I have adjusted your code (and mine in Post #13) and integrated them. There are now two CommandButtons, one to Create the list of files (cmdGetFileNames) which will populate the ListBox and the other (cmdCreateOutput) which will perform the processing on each file in the LstBox (it removes the file from the ListBox when it's been processed)

Code:
Private Sub cmdCreateOutput_Click()
Dim strFileToProcess As String
With List1
    If .ListCount > 0 Then
        For intI = .ListCount - 1 To 0 Step -1
            strFileToProcess = Mid$(Label1.Caption, InStr(Label1.Caption, ": ") + 2) & .List(intI)
            Call ProcessFile(strFileToProcess)
            .RemoveItem intI
        Next intI
        .AddItem "Files have been Processed"
    End If
End With
End Sub

Private Sub cmdGetFileNames_Click()
Dim strFileNames() As String
Dim i As Integer
Dim intI As Integer
strFileNames = Split(GetFiles, Chr(0))
With List1
    If UBound(strFileNames) <> -1 Then
        .Clear
        If UBound(strFileNames) = 0 Then
            Label1.Caption = "Files selected from: " & Mid$(strFileNames(0), 1, InStrRev(strFileNames(0), "\"))
            strFileNames(0) = Mid$(strFileNames(0), InStrRev(strFileNames(0), "\") + 1)
            .AddItem strFileNames(0)
        Else
            'If more than one file is selected the Path is stored in index 0 of array files in index 1...
            If Mid$(strFileNames(0), Len(strFileNames(0))) <> "\" Then strFileNames(0) = strFileNames(0) & "\"
            Label1.Caption = "Files selected from: " & strFileNames(0)
            For i = 1 To UBound(strFileNames)
                .AddItem strFileNames(i)
            Next
        End If
    Else
        Label1.Caption = "No Files Selected"
    End If
End With
End Sub

Private Sub ProcessFile(strFile As String)
Dim intFileIn As Integer
Dim intFileOut As Integer
Dim intI As Integer
Dim intPos As Integer
Dim strData As String
Dim dblVal As Double
Dim strOFile As String
Dim strRecords() As String
Dim strFields() As String
'
' Allocate a File Number, open, read the entire contents and close the file
'
intFileIn = FreeFile
Open strFile For Input As intFileIn
strData = Input(LOF(intFileIn), intFileIn)
Close intFileIn
'
' Create a name for the output file
'
intPos = InStrRev(strFile, ".")
If intPos > 0 Then
    strOFile = Mid$(strFile, 1, intPos - 1) & "_OUT" & Mid$(strFile, intPos)
Else
    strOFile = strFile & "_OUT"
End If
'
' Open the output file
'
intFileOut = FreeFile
Open strOFile For Output As intFileOut
'
' Split the input data into records
'
strRecords = Split(strData, vbNewLine)
'
' Process each non-null record
' If it's a record type of EC1 then split it into fields
' add whatever is in text2.text to the 3rd field
' replace the original value with the new value
'
For intI = 0 To UBound(strRecords)
    If strRecords(intI) <> "" Then
        intPos = InStr(strRecords(intI), "EC1")
        If intPos > 0 Then
            strFields = Split(strRecords(intI), " ")
            If UBound(strFields) > 2 Then
                dblVal = CDbl(strFields(3))
                If IsNumeric(Text2.Text) Then
                    dblVal = dblVal + CDbl(Text2.Text)
                Else
                    dblVal = dblVal + 0.5
                End If
                strFields(3) = Format(dblVal, "#0.##0")
                strRecords(intI) = Join(strFields, " ")
            End If
        End If
        '
        ' Write the record to the output filer
        '
        Print #intFileOut, strRecords(intI)
    End If
Next intI
Close intFileOut
End Sub

Public Function GetFiles(Optional ByVal sTitle As String = "Open files...") As String
' sTitle: Optional Title of Dialog
Dim sFilenames As String
' Get the desired name using the common dialog
Set cdlOpen = CreateObject("MSComDlg.CommonDialog")
' set up the file open dialog file types
With cdlOpen
    .CancelError = False
    .Filter = "Raw Files (*.raw)|*.raw|All Files (*.*)|*.*"
    .FilterIndex = 1
    .DialogTitle = sTitle
    ' set up Common Dialog flags
    ' same as .Flags = cdlOFNHideReadOnly Or cdlOFNPathMustExist Or cdlOFNLongNames Or cdlOFNAllowMultiselect or cdlOFNExplorer
    .Flags = &H4 Or &H800 Or &H40000 Or &H200 Or &H80000
    .ShowOpen
    ' get the selected name
    sFilenames = .FileName
End With
GetFiles = sFilenames
Set cdlOpen = Nothing
End Function
Note that it will add whatever is in Text2.Text (or 0.5) to the EC1 value for every file.