Hi

First of all click Jump Off at the top of the page on the right
Then click the button/horses NOT IN JUMP OFFS
Then add in the horses you want in a jump off (That way people can then select to challenge your horse)

If you want to challenge someone else's horse simply go find a horse to challenge (best place to look is the forums) select a horse and click the button JUMP ME then select your horse you want to challenge it against =D

Hope this helps!

To add to Pugs, there is also an alternative way to put your horses in jump offs(the first thing Pugs explained how to do). If you go to a horse's page, at the top left there is a button that says "jumpoff status." Click it, and it will enter that particular horse into jump offs. :)