Copying Data Between 2 Workbooks?

I am working on a project that requires me to copy columns from multiple workbooks to a main one. I am a new comer to VBA what I could come up with gives me an error. Here's where I am right now:

Code:
Sub Import_Click()
Dim MainFile As Workbook
Dim ComFile As Workbook
Dim RDFile As Workbook
Dim UTIFile As Workbook
MainFile = Dir("C:ModelingProcess FilesMain File.xlsm")
ComFile = Dir("C:ModelingProcess FilesModeling - Commercial.xlsx")

[code].....