Lori you stated that the filter has no name, bells should be going off and at $69.00 US when the Tiffen 77mm is $143.00 with their variable ND filter rated on the low side.
The article that Mark post has some names of manufactures of variable ND filters check out their products, I am going to add a link to a supplier in the US that I use for filters in New Hampshire. They carry a wide selection and the ladies on the desk really know their stuff as all they sell are filters.
http://www.2filter.com/
Singh-Ray variable ND filters are probably the best, but you pay for that. Good filters should last as long as your lens, a good thing to do is buy the largest filter say a 77mm then a step down ring from the filter's 77mm to one that will mount onto your lens say 67mm.
Another thing I found I use to have a D7000 and never had any problems with light leaking in on long exposures (minutes), however when I went to a D600 Full Frame light leaks in, so I have a dark cloth that I put over the viewfinder to prevent any light from leaking in.
Cheers: Allan