I was trying to export custom MPs from production environment and import to test environment. Unfortunately, I got "Failed to process extensions for management pack: Invalid enumeration mapping" error several times in our test SCSM 2012 environment.
Actually, It is pretty simple to get over this error in two quick steps;
1) Clear inside of <extensions> ...</extensions> section completely and import your custom management pack
2) After successfully imported, import the original custom management pack, therefore overwrite existing
If you have more than one MP in the same situation do the same procedure for all of them.
SCSM will generate <extensions> ...</extensions> section itself.