He asked "how", Paul. Scott, try this:

1) Open two workbooks A and B.

2) Select an empty cell in A.

3) Type this sequence: "=<Ctrl-Tab><Down><Enter>".

Observe the formula that Excel just created; it shows the syntax you must use when pointing a formula to another open workbook.

(If you're not sure what just happened: "=" started the formula. <Ctrl-Tab> moved from workbook A to workbook B. <Down> moved the cursor down one row, pointing to the next cell (probably A2, but whatever). <Enter> completed the creation of the formula.)

Next:

4) Close workbook B.

Now look at the formula again; the syntax has changed a bit, showing what you must use to point to another closed workbook.

I usually find it easier to let Excel set the syntax, and then repeat it myself (modifying it and copying it down the column or whatever is appropriate), rather than typing it out myself and possibly forgetting a critical piece of the formula. And by the way, I've noticed that it's a little slower doing lookups to another workbook, and a lot slower if that workbook is closed. YMMV.