Ok, a few things ...
Constants really should be defined outside of the scope of a procedure.
If the program is to be used locally, simply hard reference the file system object into the project i.e. "Microsoft Script Host Object Model"
You are missing a parameter in the call (to create if it does not exist)
Working example is:
Const ForReading = 1
Const ForWriting = 2
Const ForAppending = 8
Public Sub OpenTextFileTest()
Dim fs As New FileSystemObject
Dim f
Set f = fs.OpenTextFile("c:\testfile.txt", ForAppending, True, TristateFalse)
F.Write "Hello World!"
F.Close
End Sub
Remember that this does NOT append carriage returns so if you want the data to append to the next line, you should write vbCr or vbCrLf to the file.