Click on your nick [or someone else's] in the forom....and a drop-down will show....
Actually...that specific part is an admin function....though there is a 'find posts by....' as well
Yeah the find posts by is not all that helpful since most folks have far fewer posts than replies and simply using the google site search i.e.
"by Mumblefratz" site:forums.galciv2.com gives you the same thing.
In fact as far as I can tell, all the local search function does is to call google search so that you don't have to explicitly supply the site search tag. Like I said it's better than nothing but definitely not as good as the old forum search which definitely did specifically find replies by user for everyone plus had some useful advanced search features.
Using it to search for Mumblefratz on Galciv2 comes up with about 1000 entires however I've made close to 4000 replies on this site so it's clearly not complete. All in all it really adds nothing to going to google except save you the trouble of typing site:forums.galciv2.com as part of your search string.
I know beggars can't be choosers and something is better than nothing but in my opinion the old system was far superior as it seemed to be a direct search of the common forum database.