Record a macro when you do it once (This is for excel--I don't know Word!).
When you recorded your macro, you got something that looked like:
Option Explicit
Sub Macro1()
Workbooks.OpenText Filename:="C:\myfile.txt", Origin:=437, StartRow:=1, _
DataType:=xlFixedWidth, FieldInfo:=Array(Array(0, 1), Array(15, 1), _
Array(41, 1))
End Sub
Well, instead of having your filename in the code, you can give the user a
chance to pick it themselves (take a look at getopenfilename in VBA's help):
Sub macro1A()
Dim myFileName As Variant
myFileName = Application.GetOpenFilename(filefilter:="Text Files, *.Txt", _
Title:="Pick a File")
If myFileName = False Then
MsgBox "Ok, try later" 'user hit cancel
Exit Sub
End If
Workbooks.OpenText Filename:=myFileName '....rest of recorded code here!
End Sub