On the iPad, go to Settings > Mail, Contacts, Calendars;
on right side of display under Accounts, select the account name associated with your MobileMe account;
within the window that opens for that account, turn ON the switch associated with Find My iPad, then select Done.
With a WiFi-only iPad, I believe the MobileMe "Find My iPhone" feature will locate your iPad on a map only if the location (lat/long coordinates) of the WiFi router being used is registered with Skyhook (see
http://www.skyhookwireless.com/howitworks/coverage.php).