How to add a trusted path in Mathematica?

You have to go down through quite a few dialogs to select a directory path.

1. Select the System tab in the Preferences dialog.
2. Click on Edit Trusted Directories....
3. The Trusted Directories dialog appears. Click on Add.
4. A nameless dialog appears. Click on Browse....
5. A Selecte a Folder dialog appears. Navigate in that dialog to the parent directory of directory you want to add. Select the directory. Click on Choose.

You should be able to just use ~/Desktop, or perhaps /Users/Taiki/Desktop.

The interface is a little nicer through the System tab of the Preferences dialog. There you get the option to browse to a directory in addition to just typing in a path.

You can do it programmatically:

SetOptions[$FrontEnd, 
  "NotebookSecurityOptions" -> {"TrustedPath" -> Append[
       CurrentValue[$FrontEnd, {"NotebookSecurityOptions", "TrustedPath"}],
       {"C:\\Whatever\\Test", "D:\\Something\\Subdir"}]}];

though the confirmation dialog will still pop up for you to accept changes.

In any case (using the options interface or doing it the above way) you can enter the absolute full path of your directory in the way you would use it otherwise in Mathematica in your OS.