Thanks for the reply.
Double clicking the package in Unity tried to open another Unity instance, which it didn't like. So instead I copied the Playmaker package to my desktop. I closed Unity and restarted, making a new project from the splash screen. I chose this time not to add Playmaker at this stage. Once it was up, I imported the desktop Playmaker package by selecting it through the assets/import custom package menu items.
I should say that each time I try to import, I get a dialogue (pictured) - I have tried both options.
I got the same result as above - a useless submenu under Components. Errors also appeared for the first time (pictured). I imported the package a second time. This time, when the package contents box appeared, only two items were selectable (I assume the only items that were not already in the project) - playmakereditor.dll.mdb and editorstartupprefs.asset. I continued with the import, with the same result again.