@Jonathan :
Step1 : Import the following product : product-extra_threadfields.xml
Go into your Admin C.P. There is "Plugin System" open and you find the voice Manage Products. Click on this.
On the left there is a link Add/Import Product : Select from your hd the file product-extra_threadfields.xml and import
Step 2 : You have to upload with a Ftp client into your forum dir.
You have 2 dir admincp and includes in file... Put this dir in same place on your forum dir.