Since the Ecosystem consist of a lot of user submitted actions, there are also some which require additional PlayMaker packages or third party plugins, which you kind of have to figure out by their name or the error that shows up when you try to implement them.
In this case you need the ArrayMaker package (for every ArrayList or HashTable action), which you can also get from the Ecosystem or download from
here and which will solve your second error.
Because you probably don't have any experience with ArrayMaker, I'd suggest that you watch a tutorial video like
this one, before trying to use it.
The other error says that this particular action has a different class name than the file name, which means that you have to open the "ArrayListFindGameObjectsInsideCollider.cs" (or whatever it might be called in your Assets folder), look for the word that stands after "class" and make sure that the file name matches that.
That might be still a little mistake in that one action and its 2D counterpart.
If this was your first experience using the Ecosystem, than it didn't look like a pleasant one; but be assured that it's usually not that finicky, just that you immediately used one of the few actions that require additional dependencies.