If you are using stock sprint rom this .cab file will not work. Flash to a custom rom first (Energyrom, mightyrom etc.)
http://www.4shared.com/file/14228336..._5_2_3_22.html
Download that .cab transfer it to your storage on your phone and install it, soft reset.
Make sure to turn off data before installing. After installation turn on data and it will ask you for your phone # enter your 10 digit # like this "2829291234@mymetropcs.com" <--WITHOUT the quote's
If done successfully you should have working internet & MMS =)
Dont forget to THANK me!
Hope this helps