Loop through every Cell in the FlexGrid and compare its value to the TextBox value.

Code:
    
    Dim row As Long
    Dim col As Long
    Dim exists as boolean

    With Me.MSFlexGrid1
        For row = .FixedRows To .Rows - 1
            For col = .FixedCols To .Cols - 1
                 'use vbBinaryCompare for a Case Sensitive search
                 If StrComp(Text1.Text, Me.MSFlexGrid1.TextMatrix(row, col), vbTextCompare) = 0 Then
                    MsgBox "Data already exists in grid"
                    exists = True
                    Exit For
                 End If
            Next
            If exists Then Exit For 'already found the data no need to continue the loop
        Next

        If Not exists then
            MsgBox "Need to add data to the grid. The question is where?"
        End If
    End With