You might find this forum thread helpfull if you have Android 2.1 or older.
What should I do if my purchased SVOX voice is not working?
First of all, run the voice application (it has the flag as an icon and should be easy to spot in the application list). Running the voice application will install the actual voice.
Make sure the "SVOX TTS Engine" is installed and enabled.
If this didn't helped, try to close Talk To Me by killing the process or force closing. You can obtain the same effect by restarting your phone.
If none of this solution helped, write me a feedback message from the Feedback menu of the application (this sort of contact message attaches some informations about your configuration that will help in the troubleshooting process).