From help on "merge workbooks"

Make sure the copies of the shared workbook that you want to merge are all
stored in the same folder. To do this, you must give each copy a different
name.
Open the copy into which you want to merge changes.
On the Tools menu, click Compare and Merge Workbooks.
If prompted, save the workbook.
In the Select Files to Merge into Current Workbook dialog box, click a copy
that has changes to be merged, and then click OK.
To merge more than one copy at the same time, hold down CTRL or SHIFT and
click the file names, and then click OK.

Are your two workbooks shared?  If not, the compare and merge will remain
grayed out.

Gord Dibben Excel MVP