.mpkg install problems

This has been a previous topic for discussion but unresolved. I have Leopard installed on my powerbook G4 and am experiencing problems in installing software with .mpkg suffixes. The message returned is that 'there is no default application specified to open the document'. This seems to be a fairly common problem but one to which no one seems to have an answer!

The problem must lie with an installer within leopard but I do not know which application within leopard is configured to install .mpkg files or what I would need to do to reinstall this or change the settings within it!

I have used Pacifist as a short term solution -however I would be far more comfortable knowing that I do not have a gremlin within the operating system!!