To check your kart transponder number in the system you should make a passing with the kart over the start/finish or any other loop. Once the transponder is detected you will be able to see the transponder number in the "Passing History Table" under your "Session Management" module.

If you are not able to see the "Open Passing History" button at the bottom of your "Session Management" module it is probably currently not active. To activate it, go to the sessions settings page by clicking on the top right corner (your logged user name) and then "Administration"->"Settings"->"Sessions". From the list of settings make sure "Show passing table in session management" is turned on.

Once turned on you can go back to the "Session Management" page and check out your passing history.

If you need to learn more about the "Passing History Table" - checkout this article.