Control Panel functions are accessed by running the .cpl files associated with them. They are located in C:\Windows\System32\ assuming C: is your system drive.
They are run from Rainmeter by using the application C:\Windows\System32\control.exe which will take as a parameter the name of any of the Control Panel applets.
The list of available applets and their functions:
Accessibility Options access.cpl Add or Remove Programs appwiz.cpl Display Properties desk.cpl Windows Firewall firewall.cpl Add Hardware hdwwiz.cpl Internet Options inetcpl.cpl Regional & Language Options intl.cpl Java Control Panel javacpl.cpl Game Controllers joy.cpl Mouse Properties main.cpl Sound & Audio Device Settings mmsys.cpl Network Connections ncpa.cpl Wireless Network Setup Wizard netsetup.cpl User Accounts nusrmgr.cpl ODBC Data Source Administrator odbccp32.cpl Power Options powercfg.cpl System Properties sysdm.cpl Phone and Modem Options telephon.cpl Time and Date Options timedate.cpl Windows Security Center wscui.cpl Automatic Updates wuaucpl.cpl Active Directory Manager admgmt.msc
So to execute one of these applets from Rainmeter use any of the Action features of Rainmeter: