Unfortunately, ‘Interview’ is not included in the types of references for Mendeley yet. Thus, I suppose you are unable to use Mendeley to cite interviews.
As pointed earlier by Hsin-Yuan Chen , Mendeley doesn't have type interview.
But if the interview is "recoverable", which it has to be for being considered a proper reference, you can cite the source directly (web page/broadcast/newspaper article).
Unless the materials are available in one form or another, you shouldn't cite them as references. I don't think this would be accepted by most journals. If these materials are compiled by you, you could just make them available then cite them. If these are made by other researchers and you have access to them, then you could provide the methods you used to access them. Finally, if you conducted these interviews yourself as part of a study, you could treat them as data and present them in suitable format and no need for citing.