How do I access my iPad's IMEI number ?

If it's a cellular model (wifi-only models don't have an IMEI number) you can get it from its box or from an iTunes backup : Find the serial number or IMEI on your iPhone, iPad or iPod touch - Apple Support

but the IMEI is not on the packaging

Is it a cellular model ? A wifi-only model doesn't have an IMEI number.

As well as on the box, if you have an iTunes backup on your computer you can get the IMEI number from that.

Wifi-only models don't have an IMEI number.

Unless you had Settings > iCloud > Find My iPad enabled on it there isn't any way to try and locate it. If you did have it enabled you could try locating it either via http://www.icloud.com on a computer or Find iPhone app on another iOS device - but that will only work if it's connected to a network.

if you ever synced it with iTunes you can get the numbers from there

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