You need to ring them
T-mobile sales on 0800 9565057.
To avoid the T-mobile voicemail charges use the much better and free hullomail. Google it or download the app when you get an Android phone.
You can read much more about the £7.50 deal here:
I, and many others, might have switched to Orange had the tariffs and data allowances been sensible, so they really have shot themselves in the foot here.
Also note that, if you can take in an NHS/Local authority payslip into an Orange shop, Orange will give a £20 discount on the phone (some other organisations are included too).