New Cadillac Convertor installed on my 1995 Nissan Maxima but the exhaust is coming through the vent. Was it installed incorrect? The Mechanic said it was do to the fuel Injectors. One person told me that the seals wasn't done correctly. What could it be?

First off, I believe you're referring to the catalytic converter. What do you mean the exhaust is coming through the vent? Like you smell the exhaust through your AC/heat system? That sounds like an exhaust leak to me. A leaky fuel injector could cause fuel smell to come into the cabin, but I would think if it were leaking that bad you would get a misfire code. But if a new cat was just installed my money would be on a bad install and a leaky gasket.