there is actually an icon to dismount, and as far as i know you cant just have it out as a pet. you are either on it or it is put away. once you double click the horn and summon it, you should be automatically mounted. when you want to get off, bring up the action icons page and you will see the dismount icon. it is a different color from the rest (green i believe). after you dismount it, it should unsummon and dissappear until you double click your horn again.
Mount/Dismount are the same icon. Open your Actions tab in the Character menu (the one with emotes, etc.). On teh first line you'll see the yellow/green icon. Click once to mount. Click it again to dismount and the strider is returned to its bugle.
Actually I think you may have to be a certain level, to be able to ride a strider.
One time, there was this person shouting for help with mounting a strider. He saw no mount/dismount icon in that blue/white icon menu, and he was lower level than the strider, which I think is the reason he didn't have a mount option.
When I was doing quest for my dragon I heard from my friend that you have to be at least 45 to mount strider ... maybe it was written on some guide or patchnotes too, not sure.
i dont know of an exact level requiremen to "mount" or "ride" a strider, BUT i do know that i got my strider at around level 50 and whenever i rode it, it was VERY slow.....they seem to give a grade penalty if you ride one before you are at least B grade......because after i hit level 52, i could ride it and it would run at normal speed......