Because these are Excel files and not a "real" database, you don't get the full outer option. You might be able to simulat this, though.
1. Don't join the tables - Crystal will tell you that this configuration is not generally supported, but that's ok because we're going to do this with the Select Expert.
2. Go to the select expert for there report. Assuming there is one key column that the two sheets are joined on, edit the selection formula and enter something like this:
(IsNull{Sheet1.KeyColumn} or IsNull({Sheet2.KeyColumn}) or {Sheet1.KeyColumn} = {Sheet2.KeyColumn})
This should simulate a full outer join for you.
-Dell