All of reasons factory sound bank patches for the MClass devices are contained with combinator devices. You can access them by selecting 'Create Effect'. From there you should be presented with categorised folders with various patches with various different uses. (such as multiband compression, EQing, mastering).
If you would like to save your own MClass patch, just create a combinator device in Reason and create an Mclass device within the combinator, then you can tweak the settings to how you want them and then save the patch by clicking on the floppy disk icon on the combinator device.
You can find a large list of free and commercial refills on this site for download, check out the downloads section and navigate to 'Refills'. It's as simple as downloading them to any desired folder you wish, it's best to create a specific folder on your hard drive to store ReFills in, and then when you want to load them, you can search for them within reasons patch browser when having a device selected or by the aforementioned 'create effect' or 'create instrument' options you can find when right clicking.
Just don't keep your ReFills in the same folder which reasons files and data is contained within, as they will be overwritten and lost if you ever update Reason.