An even more simple version....
Actually, you can do this with 15 lines of code and process only one line at a time to eliminate any memory concerns. I've added comments to explain it as it goes.
Code:
Dim InputFileNum As Integer
Dim OutputFileNum As Integer
Dim Buffer As String
' Get the input file handle and open for input-only
InputFileNum = FreeFile
Open "SourceFile.txt" For Input As InputFileNum
' Get the output file handle and open/create the file
OutputFileNum = FreeFile
Open "OutputFile.txt" For Output As OutputFileNum
' Read each line in the input file one line at a time
While Not EOF(InputFileNum)
' Reads one line of text
Line Input #InputFileNum, Buffer
' Check to see if MAMPLASAN is contained in that line...this is NOT case sensitive
' Use vbBinaryCompare if you are looking for case sensitive
If (InStr(1, Buffer, "MAMPLASAN", vbTextCompare) <> 0) Then
' If it is found, output it to the destination file
Print #OutputFileNum, Buffer
End If
Wend
Close #OutputFileNum
Close #InputFileNum