You could also try these commands in the Terminal.app:
pluginkit -a "/Applications/DEVONthink.app/Contents/PlugIns/Clip to DEVONthink.appex"
pluginkit -e use -i com.devon-technologies.think.clipper
You could also try these commands in the Terminal.app:
pluginkit -a "/Applications/DEVONthink.app/Contents/PlugIns/Clip to DEVONthink.appex"
pluginkit -e use -i com.devon-technologies.think.clipper