Using SCOM->Nagios link
VERSION 0.1: This documentation needs to be improved.

1. Install NRDP on your Nagios server.  
This is how the plugin sends notifications into Nagios.

2. Add services like 'SCOM: General' to your Windows hosts.  
WHich services you add depend on which rules you want.
As a basic, try something like this:

define service {
        use generic-service
        service_description Passive Service
        active_checks_enabled       0
        passive_checks_enabled      1 ; passive checks only!
        flap_detection_enabled      0
        is_volatile         1 ; alert for ALL bad traps
        max_check_attempts      1 ; number of fails before alert
        notification_options    w,c,r
        stalking_options        w,c

        service_description SCOM: General
        host_name myhostname
        freshness_threshold 14400
        check_freshness 0
        check_command   ok!No SCOM notifications recently
}

You might want to enable freshness checking, if you want the service to reset to 'ok' after a day.

Add one like this for 'SCOM: General', 'SCOM: CPU', 'SCOM: Memory' and 'SCOM: Disk', plus others if you need to.

3. Install the scripts
Install scomnagios.ps1 and scomnagios.ini into somewhere, e.g. C:\SCOMNAGIOS

4. Configure a new notification method on your SCOM server.
This should use the exec connector, with a command line 

powershell.exe "C:\SCOMNagios\scomnagios.ps1" '$Data/Context/DataItem/AlertName$' '$Data/Context/DataItem/ResolutionStateName$' '$Data/Context/DataItem/Severity$' '$Data/Context/DataItem/ManagedEntityFullName$' '$Source' '$Data/Context/DataItem/AlertDescription$'

*** check this

5. Configure scomnagios.ini
Make sure all the settings are correct, particularly the settings for the NRDP access.

6. COnfigure SCOM to use this notification method
Make sure the notifications you want are send this way.  

7. Check received alerts
Your nagios.log should show which alerts have been received, and you can tune your configuration appropriately.