You access task scheduler via Computer Management (Right-click Computer, select "Manage"). When you create a task you can specify "Wake the computer to run this task" on the "Conditions" tab.
The task you need to run is called "MPCmdRun.exe" - it's a command-line utility included with MSE that lets you specify what to scan. For example, the following command will perform a full scan:MpCmdRun.exe -Scan -2
Here's some more information on MpCmdRun:http://www.thewindowsclub.com/command-line-utility-for-microsoft-security-essentials