There have been a number of similar, overlapping promos offered by Chase for UA MP members.
The general, open-admission 50k bonus was only available over the phone, and officially ended 4/30/10 (though some claim to have gotten through up to 5/2/10).
There are a number of slightly-different, invitation-only (promo code and invitation code required and must match an offer sent out by Chase) 50k bonus offers out there as well.
Unless you received an invitation, you will not be able to access those offers.
I applied for and received the open-admission card, and the 50k bonus posted to my MP account after my first statement closed. I have since received other, invitation-only offers from Chase in the mail, with offer expiration dates later than 4/30/10, but slightly different T&C from the card I actually applied for and received. The biggest difference seems to be that the invitation-only offers require a $250 spend to get the signup bonus, while the open-admission offer required only a first purchase.
If anyone has been able to sneak into the open-admission past 5/2/10, I've yet to hear or read about it, but the invitation-only offer letters do appear to still be going out.
Hope this helps.