and if by some chance they don't automatically sync plug...
- click on your ipod while it's plugged into your computer
- click on the tab that says apps
- on the left hand side of that tab should be a long list of all the apps you have purchased, make sure that the apps you just bought have a "check" next to them so that they sync with your ipod (if they are un-checked then they won't automatically sync with your ipod)
Hope this helps