To install the mpkg installer, you'll need to download the zip file located on this forum... Then, the easiet way is to copy/replace the .system file (w/c is hidden, you'll need to "show hidden folders") to the root directory of your SD card...
And to install pkg apps:
Unzip the pkg file you downloaded (most pkg apps here are zipped), then hold stylus on the pkg app and select "open with"... Then choose the pkg installer application, then put a check on "use this app to open files of this type" so that the next time you'll just need to click on the app and it will automatically open the file with pkg installer...
I think this has been discussed before and is on the sticky's...
Phone: Samsung Galaxy TAB