I just went through this problem. Texstar implied that it was fixed in the repository, so try reloading and reinstalling it. If you are not successful, the problem is that files that the program looks for in one location are in another location.
Here is a fix if you do not reinstall:
copy the files in /usr/share/xlog to /usr/local/share/xlog
copy the files in /usr/share/pixmaps/xlog to usr/local/share/pixmaps/xlog
copy the files in /usr/share/doc/xlog/ to /usr/local/share/doc/xlog
The above should make everything work, including the help file; it did for me. However, I would suggest a first try at a reinstall.