Samsnov, I think the easiest way to do this the first time goes like this. I'm going to refer to the Target workbook as "To", and the one you want to do the pointing as "Fm".

1) Open both workbooks.

2) In Fm, start a formula in one of the cells by typing "=".

3) Don't hit <Enter>; instead, leave that formula hanging and move to the target workbook, ie To.

4) Note that in the formula bar, the formula you started is still waiting to be completed.

5) Click a cell in the To workbook. Notice that in the formula bar, the formula in Fm now points to the cell in To.

6) Now you may hit <Enter>.

At this point you can examine the formula you just created to see the syntax used for pointing to a workbook that's open. But wait, there's more!

7) Close the To workbook.

8) Now look at the formula in the Fm workbook. You'll note that the syntax has changed; it now includes not just the workbook name, but the path too.

There you go: That's the syntax to use. Personally, I find it easier to create an external reference this way all the time; typing out the path and file name, and making sure I get all the punctuation in the right place, requires enough effort and proof-reading that letting Excel do it works faster for me.