You need to unpack the .rar file, this is usually done by right-clicking the file to unpack it or even better with the program Windows Commander. In the unpacked folder you will find the image pack and a file .abr That is the file you need to open in photoshop. Let me know if you need any help installing the brushes.
That is the file you need to open in photoshop.
Let me know if you need any help installing the brushes.