I’m not sure this is kosher or that it would work, but its what I do for the MODx installs I’ve made and it’s gotten the sef urls to work.
-I uploaded a .zip file called "htaccess_files.zip" download it an unpack it.
-Place the first .htaccess file in the root folder of your MODx install
-Place the other .htaccess file located in the "manager" folder inside the "manager" folder of your modx install
-Delete or rename the htaccess.txt files in your root folder and manager folder
Let me know if that worked!
Best,
Josh