I am assuming that you are on a Linux Machine? On Linux, you have to install astrometry.net, it is a separate package, whereas on Macs, you don't have to worry about that since it is in the app bundle. And on Linux, you have to configure it manually using that astrometry configuration file. KStars cannot edit the astrometry.cfg file on Linux due to permissions, so that tab is not present in KStars on Linux. Likewise, it sometimes has trouble installing the astrometry index files on Linux due to permissions and you end up having to install them yourself, but the index file installer is still there on Linux because it mostly works on Linux.
If you have astrometry.net installed on your Linux machine, you should be able to edit the configuration file in a text editor.