EDIT: JVN was faster
About the mating, you probably mated the two sides as "coincident", which means the two faces want to be in the same plane. Now, when you try to mate them at the holes, one of them wants to shift, but since the other mate won't allow it to move, there is a conflict. Change the mate type of the two faces to "parallel", and then you should be able to mate the two holes as concentric.
About the smart fasteners, you can enable the toolbox by going to Tools->Add-ins and selecting Toolbox and Toolbox Browser. If you check "Start-up" they will be enabled every time you open solidworks. I recommend doing that, as the toolbox is extremely useful (check out the gears in the Toolbox, they make my life soooo much easier
)