I know this has pretty much been solved, but should it be helpful, I'm pasting the complete transcript I had with T-Mobile customer service on how to get this to work. I'll put the quickie links at top:
Transcript:
Customer ChatChat Transcript
Please wait while we find an agent to assist you...
You have been connected to !James P.
Ahniwa : My LG Nexus 4 is arriving in the mail today. How can I get it set up with the $30 "web only" prepaid plan with 100 minutes and unlimited data and texting?
Ahniwa : Hi James, how are you?
!James P: Hello Ahniwa. Thank you for choosing T-Mobile Live Chat Support, my name is James, and for your reference my Rep ID is 145-1362. I'll be glad to check on how you can avail of that plan.
!James P: And I'm doing just great, thanks so much for asking. How about you?
Ahniwa : I'm doing good, though I had a frustrating experience in the T-Mobile store earlier. Hoping you can be more helpful.

!James P: No worries, I'll be making your concern my top priority. Basically, first of all you'll need a SIM card, to activate strictly on our website. Do you have that right now?
Ahniwa : I don't have one. That's why I was in the T-Mobile store, but he told me I couldn't buy an unactivated SIM. So I'm trying to figure out the right order to do things.
!James P: Well, you could always place an order online. It may take a few days before you receive the phone, but the plan you want is a promotional one, and you need to access this link to activate the SIM.
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!James P: The plan is also available at WalMart stores.
Ahniwa : Okay, so my phone gets here today. The only two options I have are to order online or to go to Walmart once I have the phone? Is that right?
!James P: receive the SIM rather.
!James P: That's correct. And all you have to do is just follow the prompts to activate. You must be real proud about the new phone. The Nexus 4 sure is popular.
Ahniwa : I'm pretty excited about it.
Ahniwa : Alright, so say I want to order online ... can you send me a link to the product I need to get?
!James P: I believe the Nexus 4 uses a Micro SIM?
Ahniwa : Yeah.
!James P: Here you go.
Prepaid Phones | Pay as You Go Prepaid Cell Phones | T-Mobile
!James P: This one, to be exact.
Sim Card Activation Kit For Prepaid Phone or Unlocked iPhone | T-Mobil
Ahniwa : Okay, so I order that. Once it arrives, I go to this URL and follow the steps:
Unsupported Browser -- Is that right?
!James P: You got it. And please be advised it needs to be on the website, or else, you get a different plan. You can always chat in for assistance if you need any.
Ahniwa : " it needs to be on the website" -- You mean, I need to go through the activation ON the website, right?
!James P: Exactly.