HELP!!!!Going crazy with this one.

[gxfreebies]

Here's my problem...I installed vb 4.1.11 installed the upgrade 4.2 and the security patch2. Now When I post a thread, I get a fatal error message as follows:

Fatal error: Class 'vB_ActivityStream_Manage' not found in /home/gxmtf/public_html/mountaintop/includes/class_dm_threadpost.php on line 1877

if (!$this->pre_save($doquery))

I am not a programmer so I am learning as I go. I have no idea what this line does or how to fix it. When I check to see if the thread has posted, it's there and the post count increases as it should. Think it is in the redirect but not sure. Can someone please help me fix this, and please keep it simple terms....

Thanks in advance.