One way:

With both workbooks open, type = in the desired cell in the "main"
workbook. Navigate to the "reference" workbook and click in the cell
to be referenced. Type Enter. XL will automatically enter the
correct reference, something like:

   =[reference.xls]Sheet1!A1

Now close the reference workbook. Notice that the reference changes
to:

   =<filepath>[reference.xls]Sheet1!A1

Of course, you could enter the results of the above directly, too.

This should work.
='sheet name'!cell