There are two options here really. If it's a one time scan you are trying to kick off, you can click on Subnets > Scan All Subnets.
The second option is to close you IP Address Management and open 'IP-Address-Management.IPDB' in MS Access. If you open the Subnets table, you will see a 'Scan Interval' column which can be edited. If you change this, you should be able to quickly update the timing on your subnets.
Jason, thanks for the information. You saved me about hald a day's labor. It took about 5 minutes to execute and verify.
Hope others find this useful.