To access any TBP markets open the 'Market Selection' window on either the main screen or in guardian and navigate the following path
Horse Racing > GB > then the meeting you want
and all markets including the TBP ones for that meeting will be listed below
Or you could create a 'Market Filter' to find them for you, there is a ready-made one you can download from the 'Shared Market Filters' section
viewtopic.php?f=63&t=19609