Can't load snow leopard to my mbp it is runing leopard it is saying can't load on this unit?

Does you machine meet the system requirements?

http://support.apple.com/kb/HT4949

Is this a retail Snow Leopard disc from Apple, (the one with the white cat on it, not a grey one from someone else, if it is a grey one it will not work as most of those were designed to be machine specific.

This what the retail disk looks like and you can get it here: http://store.apple.com/us/search/snow-leopard-10.6.8#!!

If it does take your original install disc 1 power up the mac and push in the disc holding the OPTION key. This will bring up the start up manager and allow you which disc to start from, select the install disc. After if finishes booting, select your language, click continue. On the next page in the menu bar select utilities/disk utility. select)highlight) your volume (default name Macintosh HD, unless you named it something else) from the left column, under the First Aid tab click verify disk, if it comes back with errors (they will be in red font) then run Repair Disk, you may have to do this more than once (untill it comes up with this disk seems ok in green font)

Quit out and eject the disc.

Then power off and power back on holding the C key, select your language and follow the prompts

All MBPs through the 2011 models can run Snow Leopard. If you are using Leopard as an OSX, the Snow Leopard shoud install on your MBP. Are you using a white retail disk or a gray installation disk? If it is the former, the disk may be dirty or faulty. If the latter, it may not ber compatible with your MBP.

Ciao.