WAN acceleration is used in Backup Copy Jobs or Replication Jobs. More info about it can be found in the userguide: http://helpcenter.veeam.com/backup/vsph ... y_wan.html
. If you create one of these jobs you will get to this point and will be able to use the accelerators.
In regards to the cache. If you have only 89GB disk space, put your cache on 70 or 80GB as we also need additional space for metadata.