CALM YOUR FARM YOUNG MAN, I'M HERE TO HELP.
The reason for the three PSD's: If all of those were put in one PSD it'd be really big and take up a TON of ram, splitting them apart makes them easier to work with. WHAT ARE THEY THOUGH!?!?!? The game uses what are called shader programs, in order to use these the models need two other "maps", one is a normal map and the other is a specular map. What a normal map does is direct the light coming toward and reflecting off the bike, it's mainly used to fake high detail 3d effects on low detail models. Here is a video explaining them with visuals:
https://www.youtube.com/watch?v=SQrHkKnSBcA . This post by Jre shows exactly what they can do:
http://forum.mxsimulator.com/viewtopic.php?f=7&t=31816
Jre wrote:The power of normal map.

Specular map tells a specific area what color, and the hardness of the reflection the light is. Here is a post that explains how to use them in game:
http://forum.mxsimulator.com/viewtopic. ... 449#p94449
For the graphics: If you look in the layers panel, you will see a ton of different folder icons which are groups, keep looking until you find one named "Plastics Parts". Click the arrow next to that one and you will see more groups appear, now look for the one named "Graphics". Expand that little guy, this is where you can put your graphics. Just keep the paint away, don't want to make a drippy mess all over.
Trying to get the bike in game, you need the actual bike model files. For MXS they're called '.jm' files, if you look a few posts up there are links for them. Here is a tutorial explaining how to name the three image files and the jm files:
http://forum.mxsimulator.com/viewtopic.php?f=12&t=1862 (note that post is a little out of date, follow that but replace the bike name with either "yz125" or "yz250". You can put the bike in game with only the normal texture, but it's not going to look good without the other two maps.
Lastly, HAVE PATIENCE, THIS CAN BE CONFUSING IF YOU'RE JUST STARTING OUT BUT KEEP YOUR JETS COOL AND DON'T GO BLOWING FUSES, THEY ARE ANNOYING TO REPLACE!