Metatrader's program folder is where you installed your MetaTrader terminal. The default location is under C:\Program Files (x86)\{your-broker-folder} or C:\Program Files\{your-broker-folder}  (depending on the version you are using). This is also the folder where terminal.exe (MT4 executable) or terminal64.exe (MT5 executable) is located. If the above does not help you in any way, please read the following:

You can quickly locate your MetaTrader's program folder by right-clicking on the icon you use to launch your MetaTrader and choosing "Open File Location" from the context menu.

This will open your MT4's program folder.

The approach presented above is demonstrated in the following custom charting installation video:

Alternatively, you can follow these 3 short steps to locate the program folder:

1) Choose "Open Data Folder" from Metatrader's File menu. This will open your data folder:

2) In your MetaTrader's data folder you'll see a text file named origin.txt. Double-click on this file to see the contents.

3) Use notepad to open the origin.txt file - the file contains the full path to your MetaTrader's program folder.
This will look something like this: