Rule: Chat
The best way to find Ladder matches is to announce in chat that you are looking for a Case's Ladder match so that other members will know you are interested in Ladder-play. You can also directly challenge a specific member in chat.

There is no rule that says an open table comes first. If a game at an open table is started before you are challenged in chat then the challenge would take precedence. If the game at the open table is started then the game must be completed, or one member can forfeit and report at any time, or both players can agree to a cancel.