My debug.txt is in my home folder: /home/TOPYWO
I use Lubuntu 11.10 (Linux)
I open it by clicking on it. There's a lot of information in it. You should look mainly at the last part, especially for ERRORS.
Do you use windows or Linux?
If you use Linux, and want the latest (working) version, I advice you to scroll down to the readme.txt on the Github-page, look under GNU/Linux and copy paste the lines in your terminal (for the libraries use "sudo" before your paste). For me that works. I once used a launcher and never tried installing it another way. It's 5-10 minutes work.
If you have windows you can use:
http://minetest.net/packages/nightly/?C=M;O=A to download the latest version. Seems to work, but I never had to try.