If you only have factory defaults set, you will need to setup an SNMP community string with su (super user) access as well.  This string has to be set (and match) on the devices and in OMNM.  If you already have SNMP configured and still don't see the devices, make sure you press the Go button on the Devices screen after discovery, since this screen does not refresh automatically. Using OMNM (OpenManage Network Manager) 4.4, you should be able to discover up to ten devices, including 6248 switches as long as you don't already have 10 other devices discovered.  You may also purchase a license for discovering devices beyond 10 if needed.      Hope this helps V
I just downloaded the OpenManage Network Administrator.  I have five 6248's that I am having trouble discovering. I've tried both HTTP and Telnet authentication methods. Both fail, which is odd, since a web browser and a telnet session (outside of OM Net Admin) will work. Latest firmware on the powerconnects. Factory defaults, except setting the admin password and IP address.    
Cannot Discover PowerConnect 6248's in Openanage Network Administrator