If your getting a little pop up message when you try to start a bot it's due to a problem with java that's been happening lately. If you don't have the latest version of java download it. If that doesn't work simply restart your computer and you should be all set
It could be what MMA said.
If not un install the client and re-download it. I know it Auto-updates but sometimes if messed up, I've had to do it before.
But yeah, try downloading the latest Java, should do the trick.