There's a couple of ways you can do this (well, three, actually):
1) Download the app from the App Store directly onto your iPad. It won't cost you again (if it's a paid app) and it's very convenient
2) In iTunes: With the iPad plugged in, go to the Apps tab (at the top, on the right hand side). There'll be a list of the apps in iTunes. Scroll down the list and when you find the one(s) you want, put a check mark in the box. When you are done selecting, hit the Apply (or maybe it's Sync) button in the lower right hand side of iTunes. It'll sync those newly checked apps over to your iPad
3) Or, in iTunes, you can simply click/hold an app out of the list mentioned in #2 above and then drag it to the appropriate screen of the iPad. You'll still have to do the sync button part from #2...
If you elect to do either #2 or #3, just be sure to do a "transfer purchases" first before dragging/checking and syncing (you transfer purchases by right-clicking on the iPad line in the left column...). That'll ensure that all the apps on the iPad are updated in iTunes as being on the iPad before you start doing any syncing.
Hope that helps.
Marilyn