I cannot connect a client to the server application. What´s going wrong?

Well, actually there are two possibilities. First of all - if you downloaded the server from Mac App Store, you actually can't. This version is free and can be used to rehearse timed speeches, group presentations, and manage meetings - and cannot be used as a stage timer in a professional environment.

Secondly - if you do have a version of presenterCue Server which should be able to connect to one or more clients, then please check you network settings. presenterCue Server communicates via Bonjour - so both, the client and the server need to be in the same network.