highscore has to be enabled in plugin.ini not in mame.ini.

Once again, the best way is to download whole official mame 178 package, extract it. Finally put groovymame into official (You can do this way too) mame folder and replace main executable if needed.
One thing, just make sure that mame.ini and plugin.ini are in ini folder. It does not matter. Both files can be in main folder together with mame.
Here is example plugin.ini
#
# PLUGINS OPTIONS
#
cheat 0
cheatfind 0
data 1
dummy 0
hiscore 1
layout 0
timer 0