I had a simlar problem with my PDA. This is how I overcame the problem:
Check that the three files (CHK, VIF and BMP files) for each voice have the same title, ie dataXX.chk; dataXX.vif, dataXX.bmp.
Having checked the files all have the same titles, turn off your Palm Treo, remove the SD card (I am assuming that your Tomtom program is stored on a SD card or similar portable device), re-insert and start the Tomtom program.
If you now look through your voice library, you should see the downloaded voices.
I was having trouble finding my downloaded voices on my TomTom One once I had transferred them. Turns out they were in the non-English area of my voices file. They still work fine though. Great forum! Thanks.