The script automatically cater for changes involving Static Platlists but NOT for AutoPlaylists. They have to be updated manually by right clicking on the Playlist node and select "Update Playlist info in Main Window". Which does the same thing as the ""Activate/Update"" button in the Options.
You activate the script in the settings:

Here's how it looks in the main window, I've used Custom1 as the field:

There are some things which aren't optimal:
- To see made changes in a playlist in the Main Window. You need to select another node and then go back to the node you were in. You can try to press F5 but it doesn't always work.
- The Column(s) you have selected to see the Playlist info in is actually written to the tag of the file. However, the script doesn't automatically write the values to the tag when you activate the script. But after the script is activated and you change something in the track properties, the value is written to the tag. (Normal MM behaviour.)
- If you don't want to use the script after you have activated it. You must uninstall it from within MM and then restart MM. You can't just delete the .vbs file since it will still leave the Database Triggers in the Database.
- If you uninstall it you also need to manually delete the values in the selected field.
Other than that I hope you'll like the script. It was a bit difficult to get everything to work correctly, especially with the Database Triggers.
Special thanks goes to Nyn who has helped a lot in testing the script!

As usual. Comments, bug reports and suggestions are welcome!
Enjoy!
/Bex
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Ver 2.0.0 (2012-08-05)
- Added support for AutoPlaylist (Idea by Onweerwolf )
- Added possibility to store Playlist info into same or different columns (Idea by wutzin)
- Added optionally exclude Playlists
- Added a nice progress bar
- Fixed bug with updating of new Playlists introduced in MM4 (Thanks ZvezdanD)
- Optimized triggers a lot
Ver 1.1 (2009-06-18)
- Added possibility to add Count of Playlists to the field
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Download:
Latest version:
MM4 (Installer)
Playlist in Main Window 2.0.0
MM3 (Installer)
Playlist in Main Window 1.1
Installation Instructions:
- New Install or upgrade:
MM3 (Installer)
Avoid "Product installation error"
- Vista/Win 7 Users:
- - To be able to install scripts you must Run MM as an administrator.
- - It means that you must right click the MM icon and select "Run as administrator" even if you are logged in as an administrator.
- All Users:
- - Check in your task manager that you only have one instance of MediaMonkey.exe running.
1. Download the .mmip file and run it.
2. Restart MediaMonkey if needed
3. Go to the settings. (If you have MM 3.1 or higher you can right click on the Playlist node, if not go to "Tools->Options->Library->Playlist in Main Window")
4. Click the Information button and read all of the information
5. Do your selections and activate the script.
6. Once activated you can right click on the playlist node and Update Playlist info in Main Window which is needed for updating AutoPlaylists.