This page is not relevant anymore since Substance Painter 2.4.
Since this version, the shelf is not copied anymore into the user "Documents" folder. Therefore all the resources should appear properly.
If the default content is still not appearing, lauching the application in "administrator" mode might help.
It can happen sometimes that the content of shelf provided with Substance Painter is not installed properly. Therefore some brushes, materials and other types of resources can be missing when you start Substance Painter. An easy way to fix it, is to simply delete a folder manually in order to force Substance Painter to reinstall the shelf.
Just try to delete the "Allegorithmic" shelf folder and then restart Substance Painter. You can find the folder at the following path :
Windows: C:\Users\*username*\Documents\Substance Painter (2)\shelf\allegorithmic
Mac OS: Macintosh > Users > *username* > Documents > Substance Painter (2) > shelf > allegorithmic