Haven't you ever played a game before? The client always has to connect to a server IP and port. MTA is just the same, putting your own IP in the box is fucking useless unless you're running a LAN server, but in this case you obviously aren't because it takes basic networking knowledge to do that, which you obviously don't have. If you haven't already figured, there is a server browser that the development team kindly built into the 0.5 client, to make it much much simpler for people like you to join servers without even touching the IP box. Here's a screenshot:
All you have to do is effortly browse through this list until you find a server you like the look of (decent amount of players, low ping etc) and then DOUBLE CLICK it and it will automatically enter the servers IP and port into the relevant boxes and even connect to it for you! How cool is that?