1. Download & Install Java  
2. Log on to games.asobrain.com  
3. Open the lobby in a new window  
4. Connecting to the server  
5. Lobby takes ages to load  
6. Connection problems  
7. I am always alone in the lobby  
8. My player name has become invalid, what now?  
9. JAR manifest requested to run in sandbox only  

5. Lobby takes ages to load

If everything is Ok, getting into the lobby will take only a few seconds. If getting in takes very long (e.g. 5 minutes), there's probably something wrong with your Java installation, possibly multiple versions are active.

To solve this, reinstall Java properly.