I have the predecessor to the SImat, the Flexispeed II. The two machines are pretty much identical, apart from cosmetic details. To change my belt, I had to loosen the grub screw which retains the main pulley and the grub screws on the back gear. The spindle then pulls out so that you can get the old belt out and a new one in. You obviously have to thread all of the parts off and back onto the spindle.
My original belt, which was new and fitted in the V grooves, more or less level with the top of the groove, slipped badly. I replaced it with a much wider one which stands proud of the grove, but grips perfectly. I can measure it if that helps.
James.