The importance of research is going to depend a lot on which dental schools you apply to. Schools like Michigan, UPenn, and Harvard are going to place a higher emphasis on research, for example, than some other schools which may be more clinic-based. It would be a good idea to contact a few dental schools and ask them if you are concerned. With that said, even the schools I've listed still accept applicants each year that have not done any research. So, while it is not a requirement, it can be a nice boost on your application for quite a few schools.