Are you on AT&T in the US? If so, I don't think you can get it to work. AT&T won't provision your phone to be able to send or receive MMS messages if you have an original iPhone.
Some have successfully tricked AT&T into making them think they have a different phone, but A) if you do that, you lose Visual Voicemail, and B) it will eventually revert back on its own again in a few hours or a few days once AT&T's automatic IMEI scanner thingie realizes you are using an iPhone 2G.
It's total BS, but if it is possible to work around or if AT&T has changed their policy on this, I haven't heard about it.
Other carriers, on the other hand, who don't particularly care what phone you are using, work just fine with native MMS on iPhone 2G. I have a brother that I helped jailbreak and unlock an iPhone 2G for who uses it on T-Mobile, and we can send picture texts to each other just fine.
With AT&T, it's not a technical incompatibility...it's just a stupid policy decision that led to enforcement of an artificial restriction.
-- Nathan