SharePoint strikes again.
Follow the article http://blogs.catapultsystems.com/aroney/archive/2013/02/11/custom-dlls-with-sql-server-reporting-services-in-sharepoint-integrated-mode/
Normally, you can find the SSRS at C:\Program Files\Microsoft SQL Server\ MSRS10_50.SQLSERVER\Reporting Services\
If you want to find the folder for SSRS integrated with SharePoint, it’s not where it’s usually located but under the /15 hive, C:\Program Files\Common Files\Microsoft Shared\Web Service Extensions\15\Webservices\reporting\bin