First, power the unit off and back on. During the initial boot-up it will show your installed expansion ROMs, it should say EXB1 or 2 or whichever ROM you have.

If it does not show this, you may have either installed it into the wrong slot or it may not be securely installed into the right slot.

Unplug the keyboard, flip it over, unscrew the expansion door, and let's check:

There should have already been a RAM upgrade in one of the 4 slots. The ROM should not go in the first slot next to the RAM, but in one of the bottom slots. The top two slots are for RAM, and the other two are for ROM expansions. If it is in the bottom slots, simply remove it and re-insert it, making sure it is securely installed.

Reinstall the door, flip it over, plug it in, and that should do it.