I recently bought a 3-inch refractor telescope from a friend last week.
When I look through the finder on the top of the scope. I can see the object of
what I want to see (M6), but when I look through the telescope all I see is dark. Am I
doing something wrong or is the telescope not powerful enough.
Finder scope misaligned, maybe? Or maybe you should remove the wad of black paper your friend left underneath the eyepiece. 
Try using the telescope (and aligning the finder scope) in the daytime. (NO looking at he the Sun). A nice terrestrial object will do.
Find a terrestrial, daylight object. Center the main telescope on it. lock the tripod to precent the scope from moving, then adjust the finder to put the object in the crosshairs.
Note that in my experience you have to align the finder quite a bit. If it was not aligned, trust me, it was way off.