Alrighty here we go!
First...you will want more than one window or tab with the forum in it. Bring up the post you want to "join" in one...and the one you want to join it to..in the other.
Click EDIT on the post you want to MOVE THE OTHER THREAD TO...or the one that will be left. You are not editing this post...this is only to get the POST ID # which can to my knowledge only be found this way. Look in your browser's URL window. Within that big long mess you should see POST ID=NUMBERHERE.
HIGHLIGHT & COPY that number. You will need it!...
Head to the other window..the one with the post you want to move....then under MODERATE choose "Join"...
Place your copied POST ID # into the box there then click Find ThreadID...
Read carefully it will say "to be aphended to..." make SURE the right post comes up title wise.
Click Join and voila! You should be all done!
The most important thing when joining posts is to make sure you have the RIGHT Post ID number...that's vital. Enjoy your new powers and remember to use them only for good...or the procurement of chocolate or alcohol!