Hi,
PlayMaker 1.8 is still in closed beta, so unless you are part of the beta list, it's not yet available.
Unity 5 current beta is not yet supported for the UI Module as well, Unity 4.6 UI module is different and therefore incompatible with Unity 5 current beta ( at the time of writing), and so it is strongly recommanded to use Unity 4.6 to work with the new UI system, which is the public and Unity officially supported version.
Bye,
Jean