How to address MEP?

In English? I would start it with "Dear Sir", or "Dear Madam".

IMHO, an MP or MEP should be addressed fairly formally.

Dear Mr X or Dear Mrs X is correct (or Dear Ms X if you prefer a more neutral title). Some MEPs may have an additional title, e.g. Sir Graham Watson, in which case you would write Dear Sir Graham.
http://www.europarl.europa.eu/meps/en/map.html

Your Gobshiteship.

In the address, you add "Hon." before the name, and "MEP" after the name.

Sorry, PASSIVE ENERGY Rt. Hon. should only be used when addressing members of the Privy Council or Lord Provosts. CMH's answer is excellent.

Dear Mr or Mrs.... don't forget they aere supposed to work for YOU not you for THEM !