Are you using MobileMe, or just storing your addresses locally. I'm going to take a chance and guess the second.
With your iPad connected and iTunes open, select your iPad from the Devices list on the left. Go to the Info tab. Select Sync Contacts. You can either sync all your contacts and groups, or just selected groups. Sync.
To prevent unwanted syncing whenever you connect your iPad, you can go into iTunes preferences, the Devices tab, and turn off the automatic syncing.
You will have to connect your iPad and sync every time you want to update changes from one device to the other.
There are other options besides MobileMe to keep contacts in sync with the iPad. Google is one. You can set up a Google account in both your Address Book, and on the iPad. I don't know if setting up Google Contacts in Address Book will sync the ones already on your MacPro. Because I already sync with Mobile Me, there is no way for me to test it. If it does not, you'd have to transfer the individually
To set up Google Contacts on your iPad you need to set the account up as an Exchange account, not the GMail one you'll see.
Setting up Google Sync with your iOS device : Mail, Calendar, & Contacts - Google Mobile Help
The one downside is that Google will not support syncing of your groups. Only MobileMe will do that. We keep hoping there will be a free, or cheaper version of it offered soon.
Oh, if you do start syncing with an online service, be sure to turn the syncing in the Info tab off first. It could cause unwanted duplicates, or worse.