Hi,
I think it's simply because the class has been renamed, get it again from the PlayMaker Menu and save your layout again.
Nothing was removed for Unity 5 as far as PlayMaker is concerned. so you should be able to track usage of variables and events from their respective browser. Yet, I have to agree that it's not ideal, the search listing isnt't returning actions that use events to send, so it's tricky to get to the source of events. typically, I use the debugger and pause when I receive an event I don't know who sent it and come back to the source this way.
Bye,
Jean