I have found multiple topics in this forum which deal with the problem of the "eval"-button in the hardware simulator being greyed out and unresponsive. The usual suggested fix is to make sure that the .hdl file in question is not utilizing any chips that are not yet implemented. However, there is a second source for the "eval"-button error. Quoting a different post from this forum:
"1) Right after loading a chip, the EVAL button is always disabled. [..] The EVAL button only gets enabled if you change any of the inputs to a different value.
2) If the chip produces no output at all, then also the EVAL button is disabled. [...]"
I write this post to give additional visibility to this information, since I myself had to spend quite some time researching the problem before I found this fix.
In addition, I think both pieces of information, especially the first one, should be added to the hardware simulator tutorial (and the 3rd edition of the book, should there ever be one), or even better, the EVAL button should not be greyed out and just work from the get go with the provided inputs. In the current state of things, a beginner may open the Xor.hdl in the hardware simulator, just to play around, and have her first interaction with the program not work. Depending on how long he takes to fix this problem, it can be a rather frustrating and demotivating experience.
I do want to note that I very much appreciate the nand2tetris series, and acknowledge that some patching around problems like this is always necessary (and a useful skill to learn in itself!); the reason I spent so much text on this minor issue is just that it is at the very beginning of a book that goes out of its way to make this stuff as easy as possible and even states so in the beginning.