Unless by Marketing you mean marketing the products you were designing, or products in the same industry, I don't think your experience as a mechanical designer will be considered really relevant. Engineering products is a whole different ball game than marketing them, so what does drafting a gearbox in AutoDesk have to do with marketing a P&G shampoo?
And this shouldn't scare you off because you gotta find out what field you want to do and then just make sure you network and do well during MBA, and forget about your pre-MBA life. I think that if you land a summer internship in the field you want it WILL count for quite a lot. I met a guy who landed a dream private equity job with one year doing commercial banking only (not really relevant), all he did was excell in the MBA classroom. Just focus on what you want and stick to it.
I'm also doing a mechanical designer stint, I postponed my MBA in order to do it. I did it because I was lacking big-company work experience period, and just wanted to find out what it feels to work a regular 9-5 job before I enter the MBA classroom.