How to include unfruitful attempts to use software that were not explored much? I wasted spent like a month or so trying to get two software libraries to work.
They partly worked, but at some point produced only unplausible results.
That might have very well been my fault.

The third one worked right out of the box and it's the one I should have chosen to begin with. The lack of problems with this library suggests that there was either something wrong with the other two, the setup, the system or whatever.

Either way, the conclusion is that I would not suggest to try the first two libraries and go with the third one, no matter how tempting it is to try the two.

I would like to include this in my thesis, because it appears to be valuable information. On the other hand, I'm not a software expert and I might simply have done things totally wrong, which is why those libraries didn't work for me. But despite some effort, I could not find documentation on what could be the problem.

Basically, what I have to say is this:


  Tried X and Y. Didn't work. Don't know why. Not recommended. Use Z instead. Works perfectly.


Is it a good idea to include such information? It's not backed up by any reference and I cannot include all the related information about the details to reproduce the problem. Partly because I don't know them and partly because that's not what my work is about. I'm also afraid that including this information will make me look incompetent.

How should I include this information? I think the best thing to do is to state that under the overall time constraints getting either of the two libraries to work was not explored any further, which is what actually happened. But then this might look like I didn't spent my time efficiently.

Maybe I'm just too much concerned that there will be only glass-half-empty people reading this, trying to see the problem in it.
I would have to disagree with the previous answers.  I would consider documenting what did not work.  I am not saying that I would definitely do it, but I am saying that ruling out the idea, out of hand, is not good advice.  I have 27 years of experience in software development and project management, and I would very seriously think about at least a short section where I would lay out what I tried.  Sounds like there are some serious holes in your knowledge of those libraries and your attempts to use them, which might make it difficult to document what you did, but I encourage you to give it a shot and run it by your advisor.
In the end, you could take the section out if it ends up detracting from the finished thesis.  But I find that kind of honest appraisal of failed attempts to be often more informative than the success stories.  I find it to be counterproductive to the community and the researcher when academia is reluctant or completely unable to admit when something did not work or the researcher had a knowledge gap.  I find the disarming honesty of "I tried and it did not work" elevates my opinion of a researcher.  But there is so little of it, that I find myself going to non-academic sources much more often than not, when I want to find answers that are real and practical.
Be careful in how you write it, and how you make the final decision to include it or not.  If you can say, I tried this library.  This is what it gave me.  This is why it was not suitable.  I tried to find relevant documentation to resolve the issues, but no such documentation could be found.  And in the end, the ability of this project to integrate a proper library is demonstrated in the use of X library, which worked.  If you can frame it like that, informative, honest, yet demonstrating your competence by highlighting, once again, that you did successfully integrate a library that worked, you could add some real value to your thesis.
The main reason for not including such an admission, in my opinion, would only be if your ability to integrate the third library was more luck than real skill, if it did not demonstrate a concerted effort aided by much better features and documentation.  If that is the case, then forget what I have said and go with the other answers' advice.