DearElder.com has a service to write to missionaries in the MTC for free. Messages are printed and delivered to the missionary daily. From the DearElder.com homepage, click on "Provo MTC (FREE)" on the left side of the page.
You will need the following information to send your message:
Sister Kasey Nelson
MTC Box#: 113
Mission Code: CHN-HK
Estimated MTC Departure Date: 11/28/11
Missionaries stay VERY busy in the MTC. Her free time is very limited, but she will LOVE to get mail. Let me know if you have any questions.