To be able to ride your digimon, there are two conditions available. First, you must have an item called "Rider Wing". You can buy this item at Riding Agent, Megumi Shirakawa in Yokohama Village.
Second, your digimon's Riding Function must be
active. This condition is only for ultimate and mega stage. To know more about which digimon can be ride and how to active it, visit "How to Active The Riding Function".
Use Rider Wing to your digimon by right click the item. Now the riding function is available and you can ride your digimon by left clik the Riding Function Button above your skill shortcut. Happy Riding!