Can I run a Schwalbe Marathon 26" on a Mavic CrossLand UST rim?
If tires are nominally compatible with the rims, it may still sometimes frustratingly hard to put them on. There are several tricks that helped me on many occasions.


Make sure that tube does not get in the way. Sometimes it means using a narrower tube. In your case, having a tube marked for tires 26"×2.0" inside tires marked as 26"×1.5" will add extra to the torture (the tube will be "pushing" a partially installed tire away). Instead, find a tube marked as 26"×1.5" or approximately so.
Use liquid soap on tire's bead before installing. This would remove extra friction along the circumference of the rim.
Keep an eye on the first bead. Installing the first bead into the rim is usually not hard; it is the second, final bead of the tire that is stubborn. When installing it, keep an eye on the first (already installed) tire's bead. Try keeping it pushed inside, closer to the centerline of the rim, rather than allowing it resting on the hooks of the rim. This way, the tire will relieve a bit of maneuver for the second bead.
Do not rely too much on your tire levers. It should be possible to install a tire without using levers, only with one's fingers, a lot of swearing and sometimes toes to give a final push to the last 5 cm of the bead.
Sometimes, if nothing helps, it is worth starting over by releasing the bead completely. This way, you'll have a second chance for the bead to sit properly right from the start. Keep an eye on the first bead when pushing the second one!


Good luck!