According to my experience, GMAT Plus is nothing but the actual GMAT Paper Test Questions, so it doesn't have the explanations, as the paper test available on MBA.COM also don't have any explanations.
If you are finding it too easy( especially the quant part) you can easily skip first 10 questions and then try the rest of them. But you won't get the passages and critcical reasoning as those on GMAT plus, anywhere else as these are the actual GMAT passages and Kaplan
etc just gives you sufficient practice and a feel of what it could be like, but GMAT plus is the REAL THING!!! so my suggestion is first get your fundamentals in place and then take these timed tests strictly to see where u stand and use them judiciously coz you wont get a better testing ground than this so incase you feel you are not doing well master your fundamentals and then try again.