If you just installed the emulator in the last few days, we accidentally put out a bad version. We have fixed the problem, but you will need to do a "hard reset" to get the fix.
In App Inventor go to the "Connect" menu and select "Hard Reset". This should reset the state of the emulator. When you attempt to use the emulator after the hard reset, it should offer to install the new companion. This version, 2.30, should work.
Sorry for the inconvenience, and let me know if this works for you. Thanks.
-Jeff