If Exchange is not installed then extensionAtribute13 is not in the schema. Of course, without Exchange I don't know how valuable a public group is to begin with (unless you are syncing the object to another forest that does have Exchange). You have a few options:
1. Add extensionAttribute13 to the schema (yuck).
2. Extend the schema using the Exchange schema extensions (better).
3. Change the attribute WebDir uses to store this information (use the Settings tab in System Manager). The problem with this one is that there are not a whole lot of attributes available in AD out of the box.
4. Hybrid: add an attribute the schema called WebDirSecurity and set WEbDir to use this attribute instead of extensionAttribute13.
Number 2 is the easiest, you just do a forest prep but this may not be viable for you.