1. You set up how often the program will run it's cycle. When it does, it will buy the items you specify if they are below the price you set and you don't have enough. It will then check the items you have for sale. If an item is sold out, it will create a new sell order with the stock you have. If someone has undercut you, it will lower your price (unless it's hit the limit you've set). So, you set the items you want it to buy and sell, and the limits of how much to buy/sell them for. It then automates the process that you would be doing manually.
2. Depends on how many buy/sell orders you have, and how good the market is that day.
3. The program has a market analyzer to help with this. It just gives you information, and you still have to figure it out. You also have to do your market research and figure out what works. If figuring out what to buy/sell was easy, everyone would do it.
Read over the EveTrader section in the manual:
www.macrolab-online.com/knowledgebase/index.htm
It can be found in Eve Pilot>Eve Pilot GUI Overview>Eve Trader Settings