Marketing -> Portfolio Management

Internships are as much about getting experience in the work force and building a network as they are anything else. No matter what, you have to be the absolute best you can possibly be. Crush any task handed to you, learn skills that will be helpful, contribute wherever asked and generally impress whomever you are working with. That's step one. 

Step two - build your network as much as possible. You are an intern, thus it's a lot easier to reach out and ask for some time to learn more about what other groups do. I'll often work with our interns to swap to other groups, shadow roles they are interested in, etc. Show that you are interested in portfolio management, investment management, research, etc. and do what you can to learn what they do, ask good questions when the opportunity arises, etc. 

Step three - shift your thinking on this. If you start complaining to other interns, other workers, etc. about 'Oh I got placed in Marketing' - you'll torpedo yourself. It may not be your ideal situation but it's an opportunity for you. I fully respect, and understand, that it's not many people's preference - wasn't mine at one point either where I ended up, but it worked out well for me in the long run. 

Step four - learn about marketing financial services. Even though you aren't going to be in marketing long term, there's a lot you can learn about positioning and strategy. The old joke about marketing is that it spends more time marketing itself than the firm - and that might be true in some cases. But having an understanding of how copy is written, how to think about what will or won't appeal to your buyers, who the firm's client base is, etc. will be helpful. At worst - use it as an opportunity to improve writing, editing, copy skills - think through what will help you communicate whatever investment decisions you are making.