You might have a look at this thread: Activity Feeds Error on UR12 (assuming you are on UR 12 or 13)
Sounds like the issue is the solution posted on the Marketplace is outdated. Had Activity Feeds been installed prior to UR12, that process would have made an update that resolved the issue.
(taken from the link above)
1. Download and install the version of Activity Feeds on the Microsoft Marketplace / Pinpoint (version 1.0.1.20302)
2. Download and run the UR12 Server update from the Microsoft download site on the CRM server, you only need to get as far as the extraction complete / welcome screen not all the way through.
3. Use windows explorer to look through your hidden temp files for *activityfeed* and it will come up with a couple of CAB files ('ActivityFeeds_1.0.1.CAB' and 'ActivityFeeds.CAB')
4. Import the 'ActivityFeeds.cab' as a solution and it should tell you that this will update the one that is already there. The version number of this new one should be indicated as 1.0.3.21214
5. Make sure that you overwrite existing customisations (best to test this in a dev environment before doing this on any form of live solution as you can overwrite changes you have made elsewhere with this!)