Drag the two files you extracted from the zip file into the Keyboard Layouts folder.Go to the Macintosh HD (or whatever your system hard drive is named) and find the following folder path: System / Library / Keyboard Layouts.
To install the keyboard layout and icon, please follow these instructions, available from Madan Puraskar Pustakalaya: In June 2009, Madan Puraskar Pustakalaya in Kathmandu, Nepal, has updated their 2004 Nepali Devanagari keyboard layout for Apple Mac OS X. THL Toolbox > Fonts & Related Issues > Nepali Fonts > Devanagari for Mac OS X