graphicxtras.com > Photoshop brushes - how to load and install and open the ABR tutorial CC 2015 CS6 CS5 tutorial
You can load them in a number of ways. You can also install them in a number of different locations. Place the ABRs into the presets folder in the applications / Adobe Photoshop (or program files / Adobe Photoshop) location. You can also install them in a user location as the user location will not require any password / admin account to be entered. On the mac, the user location can be found via the ~/Library folder (use the finder go to commnd to get to that) and then application support and the sub folder beneath Adobe and the current version of PS and a sub folder of that - presets and brushes. If you add them in the user account, the items will appear in a slightly different position in the presets list (at the bottom) for the strokes.
If you decide not to add them into the presets location then you will have to use the load / replace commands found in the presets palette to find them. Personally, I use the load / replace commands as it means the ABR files are all located at a central location for all the 1000s of ABRs but you can use whatever is easiest. OK, once you have added the ABRs to the required location (or not) then you can get to the loading of the ABR.
You can, in some versions of PS, use the file open command to open and load the ABR and you then can open them from any location. This does not work on the mac. Alternatively, if you do not have any open documents, you can drag a single or multiple ABR files to the application and they will be added to the preset panel (this is probably the easiest way to load them on the mac)
You can just go to the ABR file and double click them and that will load them into PS though it depends on the file associations. If the file association is set to load Illustrator with ABR files then ABR will start up instead. In most cases, you should have no problem but I know I always end up with file associations with odd versions of PS or PSE and they start up instead and that can be frustrating. You can also use the right click and select open with command from the context menu and then select the correct version of PS to use.
You can load them or replace the existing set with the new set via the preset panel right side menu with load and replace commands. You can also do much the same via the edit menu preset manager commands though the menu / buttons are slightly different in their layout.