The filesearch module only works on Linux. If you installed the Debian packages you don't need to do anything. Everything is configured automatically. If you manually installed Group-Office you'll need to install some additional software. The filesearch module uses some command line programs for file indexing. Make sure the following commands are executable:
Also make sure PHP5 has the php5-xsl extension installed.
The search index must be updated in a cron job. You can edit the "Filesearch index" cron job at Start menu -> Manage system tasks. It's recommmended to run it once at night.