How do you get a monster in your party in monster ranch

Click home. Go down a little on the screen and you will see at least one monster. Below that you will see two selection bars. Click on the arrow of the first selection bar. Select an "Empty Slot" (Select a monster if you wish.), then click on the other arrow. Select a monster you have already caught. Then click on the "Swap" button, and, bingo! You've got another monster in you're lineup!