It's possible you installed the 32-bit client on a 64-bit machine, which is fine, but requires you to use our "Data Compare for Oracle 2 (32-bit)" executable.
If not, then it would be good if you could try going to "Tools->Application Options..." in the menu, and checking the box for "Prefer direct connection". Restart the application and try again.
Let us know how you get on!
Product Manager - Oracle tools