there were prolly just issues with the site. i checked it just now,and it seems to be working. just make sure you use the files linked above,as the recovery listed in the rezound guide is not compatible with the sensation. tho all the steps are the same.
hopefully the process goes smoothly once the site cooperates
if youve gotten data to work,you should have working mms also. it may just be working slowly because of your slow data speed,or its possible that youve entered apn info not quite correctly. try and determine if your using the correct apns for straingt talk using att towers,and then double check the settings youve entered. sometimes a reboot will help,and sometimes you can just use a 3rd party messaging app(like handcent,or gosms) to sent a few mms and then the stock app will start working.
the bad news,is that youre stck with the crappy data speed. assuming that the straight talk on att's towers uses the same frequencies for 2g,3g,4g as att,you will only get those speeds with an att device,due to the physical radios the device is equipped with. a t mobile or cin-bell branded device just does not have the correct radios to get anything other than edge data(the little E in the status bar) when used on att service. i mess with alot of htc phones,and have t mobile for gsm devices. whenever i use an unlocked ATT device,i only get E data,for those same reasons. t mobile and att are unfortunately different
so before you get too far into this,you may consider trading or selling your t mobile sensation,and picking up a dedicated att device. the inspire 4g is a nice att phone. it is basically the same as the desire hd,and thus,a fair amount of regional stock software is availble,as well as numerous custom roms. its an att phone and thus should get all data speeds straight talk offers.
the att one xl is a really nice phone as well,nice screen,thin,and is also a deicated att device. only prollems are that the most current software is unrootable at the moment,and youll have to cut down yuor sim to micro sim size,and if the device needs to be unlocked to use on straigt talk,then you may have to pay for an unlock code(i just did,to use mine on t mobile
)
an international phone may work for you as well,if you can find one. a non t mo sensation,or an international desire hd,may not get 4g,but usually international devices will have the correct radios for 2g and 3g. wich is better then edge
hope that helps some,just holler if you have more questions
edit
if you are near dayton ohio,or want to ship the phone,id be happy to help you.i can help with just about any htc phone that you may come across
if you can find one for a price youre willing to pay,i think youd be very happy with it.