Problems connecting to an event
Restarting the client can help in some cases, but if you get this repeatedly it's usually a firewall/router issue, darkwind needs ports 28000-28005 open (possibly higher than 28005 depending on the amount of event servers) on both UDP and TCP (not sure if TCP is really needed)
Problems running the game
Make sure your video card drivers are up to date.
On Windows, if you don't have the Microsoft Visual C++ 2005 Redistributable Package already (possibly from installing another application) then download and install it from this link:
Problems patching the game
If you're running Vista you should be running the game as administrator (right click on the .exe and choose "run as administrator"). Otherwise the patcher might not have write access to the files/folders it needs to update.
Problems loading the Lobby
If you can launch the client, and it appears to connect properly, but the Lobby interface doesn't load completely, you can check the port it is using by checking this web page:
The Lobby should be finding the proper port automagically, but if it doesn't you can enter the port reported by the above link in the Port field in the Lobby, and that should resolve the issue.