Regrettably, Ive come to the conclusion that the journey's not possible by public transport. The FC Dallas website makes no mention of public transport:
http://fc.dallas.mlsnet.com/t104/stadium/#dire ctions
(The Ticketmaster website has he same information - or the lack of it).
Additionally, the City of Frisco website states that the only public transportation is an hourly free bus service around the city (which doesn't seem to go near to the stadium).
The Greyhound website doesn't list Frisco as a destination.
There's no airport at Frisco and looking at map of the Texas rail system doesn't show any nearby stations.
Even asking someone with local knowledge would seem to be difficult. FC Dallas don't seem to have an e-mail address (but you could try phoning):
http://fc.dallas.mlsnet.com/t104/load.jsp?sect ion=about&content=contact
The Frisco Convention & Visitors Bureau also don't have an e-mail address. (Once again, you might want to try a phone call):
http://www.friscocvb.com/contact.html
Even the Dallas Convention & Visitors Bureau don't provide an e-mail address (but there's a phone number for the Tourist Information Centre):
http://www.dallascvb.com/visitors/contact.php
Chris