if you want this, you might have to change your number. it's supposed to be for new customers through wal-mart only. there are sometimes ways to get around this though.
easiest way would be to buy this sim card kit from wal-mart.
http://www.walmart.com/ip/T-Mobile-SIM-Kit/24099996
then activate it in store with a new number telling them you want this plan.
http://see.walmart.com/t-mobile/?povid=cat1105910-env542259-moduleA110913-lLinkLHNShopbyCarrierTMobile#plans
scroll down and you will see the $30 100 min/unlimited text/data 5GB 4G
i don't think you can still be in contract with your T-Mobile phone on this though.
if you want to try to keep your number (NOT Guaranteed)
buy the sim card kit from wal-mart. then call in and speak to T-Mobile customer service and ask them to switch you to this plan.
this is copied from T-Mobile's support website for proof
"We can switch you to that plan if you purchase the activation kit from Walmart and contact us with the code. Do not activate the new kit on your own as we have to do something special with it to activate you if you want to keep your same number. If you want a new number you can purchase the phone and kit in T-Mobile.com and as long as you activate your service on our site, you can select the $30/Month plan with 100 min and unlimited web/text."
the way i did it 2 years ago was this
order the sim kit from the T-Mobile website, or walk into a store and buy one.
then, go online here and setup/activate your sim online.
http://prepaid-phones.t-mobile.com/prepaid-activate
while going through the activation it will ask you which plan you want. the $30 plan should be offered there. there are 2 different $30 plans. make sure you pick the right one. you only get one shot at it. pick wrong, click activate and you'll have to buy a new sim and start over again.
hope this helps you and anyone else wondering.
J.R.