Have you tried registering any other system folder? The Introduction to GeoEvent Processor tutorial chose to use C:\arcgisserver as the base folder only because that is the default server directory used by ArcGIS for Server, and we discovered that permissions for the OS-level user responsible for running the ArcGIS for Server and GeoEvent Processor services needed to be added to 'My Documents' folder when attempting to register folders such as C:\Users\<user>\Documents\ArcGIS.
Try creating a folder C:\GeoEvent on the local HDD and registering that folder. The user you've logged-in as should be the owner of that folder. If the folder fails to register with GeoEvent Processor as a Data Store, try adding the OS-level user responsible for running the 'ArcGIS GeoEvent Processor for Server' service as a local user with 'Full Control' over the folder and any sub-folders.
/RJ