You need to login as the host user, Go to:
Admin menu / site settings
advanced settings
host settings
Site Log History in Days - enter the number of days you wish
It also sounds as though it is setup to store the site logs in the file system rather than the database which is why it is showing you that message.
If you wish to view the reports, I think you will need to use the database as the site logs storage.
To check your settings go to the host menu / HostSettings / advanced settings / other settings
SiteLog storage - select database
You will need to make sure that your sitelogs do not grow too large, for configuring this have a look at:
Site logs, Event logs and Scheduler