Can't find any version of this plugin for 1.8
Can you be more specific obout which plugin you are talking? (perhaps a link to the 1.6 version)
I had a look at the plugin, but it's a lot of work to find out where it has to be changed for MyBB 1.8. Perhaps the author will come out with a 1.8 version in the near future, you could approach him or post a message in his thread.