This works for me:
/ip hotspot profile
set X login-by=mac,http-chap
set X mac-auth-password="password"
set X radius-mac-format=XXXXXXXXXXXX
Then enter the mac address as the username in your database as XXXXXXXXXXXX and the password as "password", changing the XXXXXXXXXXXX to the mac address and password to the password. Don't use "password" for the password.
edit: I changed the radius-mac-format because my radius server software did not like the colons in the username. You can leave that alone if your radius server is ok with colons. Use the colons in the local database (/ip hotspot user) in the format XX:XX:XX:XX:XX:XX