Using Multiple Devices With the Play Store
The first thing you need to do is to check your tablet version. My suspicion is that it isn't running Honeycomb (Android 3.0+ or the tablet version of Android) but actually a heavily modded phone version of Android (i.e Android 2.1-2.3).

Go to settings -> about phone (or something similar. each manufacturer changes how you access this)

If it is indeed running the version of Android meant for phones, then the market might be complaining to your tablet because you might be trying to download apps meant for honeycomb. 

The online market automatically updates to include additional devices when you activate them (i.e when you initially do the android setup and sign in to your google account). Make sure you select the drop down and check if there is another option. Also make sure you activated both devices under the same account.

-I know I should have made this a comment but its too long of an answer for one. :P