On rare occasions, I have ran into this and I always seem to forget what to do then this happens.  It’s actually quite simple.  All that you need to do is run office diagnostics.  First things first, close InfoPath.  Then, follow the directions for your version of Windows.

Windows 7

1. Windows
2. Control Panel
3. Programs and Features
4. Find your Office installation and right click on it
5. Click Change
6. Click Repair

Windows XP

1. Start
2. Control Panel
3. Add or Remove Programs
4. Find your Office installation and right click on it
5. Click Change
6. Click Repair

Once the repair has completed, open InfoPath and check your form.  The contact selector should be working fine now.