The begin with, the last rating for Tyre on Mac is "non functional". The page for Tyre is here https://www.codeweavers.com/compatibility/crossover/details/tyre
Obviously, the last test is rather old, but in this case, I think the problem is the same with all versions. That's why there's a trial, so you can test things fully before buying, and then, there's a database to collect user's information.
That being said, you can look at the wiki here: http://www.codeweavers.com/support/wiki/mac/faq/usbtoserial
That is the problem, getting your Tomtom to work through USB, which has a very limited support. Personally, I never managed to make use of a USB device, like my Garmin GPS with Crossover, so don't get your hopes up. In fact, if it did work, you would be one of the lucky few. What is important to notice is that Crossover supports running Windows software, not Windows only hardware. To this day, I still use a win7 virtual machine on virtualbox to deal with my Garmin, I suggest you do the same for your Tomtom.
If you get things working, good for you. Otherwise, I would open a support ticket and explain the situation. The folks at Codeweavers are a pretty great bunch and I'm sure they'll find a way to satisfy you.