okeydokey,

do you start in Sub Main(), or in a Form_Load()? If it is the latter, I suggest you make your startup object Sub Main (Project menu -> Properties).

Then, you can parse the return value of Command$() in there.
VB Code:
  1. ' In a .bas module
  2.  
  3. Private Declare Function PathFileExists Lib "shlwapi.dll" Alias "PathFileExistsW" ( _
  4.     ByVal pszPath As Long _
  5. ) As Long
  6.  
  7. Private Sub Main()
  8. Dim sFilename As String
  9. Dim sBuffer As String, hFile As Long
  10.     sFilename = Trim$(Command$())
  11.  
  12.     Load <MainFormName>
  13.  
  14.     If (PathFileExists(StrPtr(sFileName)) = 1) Then
  15.         hFile = FreeFile()
  16.         Open sFilename For Binary Lock Write As #hFile
  17.         sBuffer = Space$(LOF(hFile))
  18.         Get #hFile, , sBuffer
  19.         Close #hFile
  20.  
  21.         <MainFormName>.txtTestFile.Text = sBuffer
  22.     End If
  23.  
  24.     <MainFormName.Show
  25. End Sub
That should work.