Are they the genuine SEAT accessory ones or aftermarket?
I have some Automaxi ones and they screw into four threaded holes in the roof gutter area, one above each door. If you look along the black strip at the edge of the roof you will see 2 lozenge shaped covers on each side - they have a slight angle on one end. Push this angled end down and the cover should pop upright. You can then see the thread so it's just a matter of screwing the roof bars down.
If they are anything like mine they take a bit of adjustment where the foot fits to the bar to get the spacing right for the bolts to go in, but they go on eventually. The foot also bolts through a thick rubber pad to protect the car, but this means that the screws will not tighten right up, you just need to get them tightened enough for the bars to feel solid on the car, so that when you rock the bar the car rocks on its suspension.