There are several ways to use Minnowbooster to make money depending on how much and what currency you have to invest. Two ways that I use it:

1. When I had more Steem Power than I knew how to use, I could lease it to other users through a market that Minnowbooster runs. So instead of having 5000 SP, I was getting 4.825 STEEM per day.
2. Minnowbooster buys other people's voting power at a discount and resells it. So you can use their bot to buy an upvote that will give you an 18% return. (For me it's 25% because I'm on the whitelist.) It's somewhat complicated but very worth taking the time to understand.