Actually the STK is not available for 1st generation WP7 devices. Find out more here.

http://windowsphone.uservoice.com/forums/101801-feature-suggestions/suggestions/2282839-sim-toolkit-stk-

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From: "Dennis Kioko" <dmbuvi@gmail.com>
To: "Skunkworks Mailing List" <skunkworks@lists.my.co.ke>
Sent: Wednesday, October 10, 2012 10:59:50 AM
Subject: Re: [Skunkworks] MPESA on WIndows Phone 7

Not all WP devices were created equally, Nokia's have the SIM toolkit which may be totally absent in others. 
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