Where is the 14 digit imei # on my ipad?

An iPad will only show the IMEI if its a cellular model. Wifi-only models will not have an IMEI number and cannot be added to a cellular service plan.

What model of iPad do you have exactly? Settings->General->About->Model?

Settings > General > About > IMEI

Also see: Find the serial number or IMEI on your iPhone, iPad, or iPod touch - Apple Support