There is a great article on sealed and unsealed management packs here so I'm not going to repeat the same information. Although the article was written for Operations Manager 2007, it is relevant to Operations Manager 2012. Indeed, even the process at the end works fine.
If you have Visual Studio 2013 installed though you can also create the key from there.
1. Go to C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\Tools\Shortcuts
2. Right Click Developer Command Prompt for VS2013 and select Run as Administrator
3. A help file for sn.exe can be found here. We need to create a Key File so type:
sn.exe -k c:\temp\SCOMKey.snk
4. Store the key in a secure location that you can get to when you need to seal a Management Pack.