A "Redeye"! :) How wonderful to have an heirloom like that!!
Looks to me like your belt needs to be tightened...easy enough, undo the "clamp" (wire), snip off what you feel you need to, (about an inch, maybe?) and use an awl to poke a hole in the end you cut, and reattach the clamp.
Go here:
http://needlebar.org/main~nb/ and look for a manual...it's going to be either a Model 66 or Model 99... (I'm not up enough yet to be able to tell you which it is!) but the manual will tell you where all of the points are that need to be oiled. Clean any lint/dust/etc., oil it up, shorten that belt and go to town!
There's only a small "learning curve" to using a treadle. You're going to love it! Congrats on your acquisition of a wonderful heirloom to take care of... (I don't believe that "we" own these things personally, but are instead caretakers for the family! :) )