Thanks for the information about the schemes, I imported the SQL one, but again, I ran into some problems. In my radiusd.conf, there is no authorize section and also no accounting section, so I just pasted them under the last section. When I start freeradius in debugmode, it returns the following:
/etc/freeradius/radiusd.conf: Failed to find module "sql".
/etc/freeradius/radiusd.conf: Errors parsing authorize section.
Errors initializing modules
What can be done to solve this?