How to unpack MOD?
How to open a .pkm file? If I want to translate

What should MegaMod 8.01 do?
You don't need to unpack the .pkm (which actually can't really be done, once a mod is all wrapped up in .pkm form, that's it, it's in its final form).
Instead, you need to get the StringTables, translate them, and then pack them into a mod that will have to be loaded above MM in your modlist in order to overwrite MM and apply the translated names and tooltips on top of the mod.
The stringtables for MM8 are available here:
u have to unzip zip files and unblock files. pkm's move to the banished WinData folder. start game and enable mods, EXIT GAME COMPLETELY after the game reloads with mods enabled. then restart game. failing to exit and restart will result in crash.
The question wasn't about unzipping and installing mods, but about opening the mod's code to access it and translate it I think Stiles Wink
Possible problems with the PKM format files

The inability to open and operate the PKM file does not necessarily mean that you do not have an appropriate software installed on your computer. There may be other problems that also block our ability to operate the Pokemon Saved Game Data Format file. Below is a list of possible problems.

Corruption of a PKM file which is being opened
Incorrect links to the PKM file in registry entries.
Accidental deletion of the description of the PKM from the Windows registry
Incomplete installation of an application that supports the PKM format
The PKM file which is being opened is infected with an undesirable malware.
The computer does not have enough hardware resources to cope with the opening of the PKM file.
Drivers of equipment used by the computer to open a PKM file are out of date.

If you are sure that all of these reasons do not exist in your case (or have already been eliminated), the PKM file should operate with your programs without any problem. If the problem with the PKM file has not been solved, it may be due to the fact that in this case there is also another rare problem with the PKM file. In this case, the only you can do is to ask for assistance of a professional staff.
.pkm mod files for Banished are not meant to be opened. If you want to "open a mod", you need to get the files from the modder before it gets compiled into .pkm format by the modkit.