Robert I am going to assume "pic. rail" means Picatinny Rail. The red dot should just slide on the Picatinny Rail from the front of the rail (Assume rear to be where the rear sight is). You will have to remove the screws from the site base and replace them once you have the site on the rail and tighten them down.

It will fit and work jut fine I have one on my 22/45

Robert if your Mark 2 has the Weaver rail, the Weaver micro dot sight B will install simply by loosening the attachment bracket by screw - attaching to the rail - and re-tightening. It is a snap to install and will require only an allen wrench.

You should have received a weaver mount and screws with your firearm. Unless you purchased used in which case these items may be missing. If they are missing you will need budsgunshop.com/catalog/advanced_search_result.php?keywords=Ruger+22-45+mount. To install you will need a small screw driver to remove the 4 screw hole fillers located on the top of the 22-45 receiver. The Weaver mount can be positions so the rear site does not need to be removed. Use the mount screws (ones with heads on them) to screw to mount the base to the receiver. A little thread locker helps a lot on the threads so the mount does not come lose. If no thread lock is available make sure to take an appropriately sized screw driver to the range the first couple of times and tighten the screws back up. The threads on the mount screws are FINE threads so be sure not to try and crank them down, you'll just damage the screws.
Here is a video I found to give you the basic concept on mounting optics on the Ruger 22: youtube.com/watch?v=SxbrwuBm5OU
Post if you have more questions