i know that i want to spend about 50 usd, which is about .08 btc at the moment. i dont know how to figure how much mintcoin that is and i dont understand the rate.
I see, so you want to buy MINT with BTC. To do so, you need to input two fields on the left box "buy MINT with BTC".
The second field "Rate" is the price you want to pay for 1 MINT (in terms of BTC). You can make an instant buy order with 0.00000027 (as shown in your screenshot), but you may set it to a lower number as well. It won't be executed, until someone is willing to sell at that price.
The first field "Amount to buy" is the amount of MINT you want to buy. Say, if you are going to buying MINT at the price 0.00000027 with all your 0.08 btc, you should enter 296296 (=0.08/0.00000027), and you should see a number very close to 0.08 shown on the "total" field.