First check to make sure the Apps are on your computer. Click on the Apps icon under the Library heading. Confirm they are there.
Second confirm that your iPod Touch is compatible with the Apps. Which generation iPod Touch do you have?
ID your iPod if you don't already know:
http://support.apple.com/kb/HT1353
Check to make sure you meet the minimum iOS for the Apps. Click on your iPod under Devices while it is connected, click summery, look at version number.
If all of the above is correct and you meet the requirments for the Apps, click on the Apps tab next to Summery. Check Sync Apps. Check the Apps you wish to add. If you wish see if "Automatically sync new Apps" is check (if not checking this will automatically add news Apps baught through iTunes every time you Sync/Back up your iPod Touch.
Also make sure your iTunes software is up to date.