Knowing the IMEI number will not help you locate the iPad. If you set up the Find My iPad service in the Settings prior to the iPad going missing, then there is a chance that can be tracked, wiped of its data or locked (which does the same thing as activating the screen lock), though it depends on a number of factors: the iPad is turned on, is connected to a WiFi or 3G network, and has not had the Find My iPad service turned off or the iPad completely restored. Log into icloud.com and go to the Find My iPhone section.
If you did not set up Find My iPad, then there is no way to track it. Report the loss the police and change any passwords for any online accounts you may have used from your iPad (the iTunes Store, for instance).
If you need to know the IMEI number for your cell carrier, some suggestions are in this support article:
http://support.apple.com/kb/HT4061
There's no way you can look up the IMEI number from the serial number yourself, though it's possible that Apple Support might have that recorded somewhere if you call them.
Regards.