There are two options when it comes to installing, both require you to have XBMC installed on your device. This addon is only designed to work on the XBMC platform (soon to be renamed Kodi) and cannot be run as a standalone app.
Option 1: Use Total Installer (the simpler method for newbies)
1. Install Total Installer using the guide here
2. Click on "Search by Addon/Author" and type in "ontapp.tv" without the quotation marks. You may find more than one result appears, the one you need to install is simply called OnTapp.TV
Option 2: Install via the repo download (the original method for installing addons)
1. Download the OnTapp.tv Repository (DixieDeans-XBMC-Repo) from here.
2. Copy the zip file to your XBMC device.
3. Open XBMC and go to System > Add-ons > Install from zip file...
4. Navigate to where you downloaded the repository zip and select it.
5. XBMC will install the repository.
6. Navigate to: System > Add-ons > Get Add-ons > DixieDean Repo > Video Add-ons and install OnTapp.tv from the repository.
If you get an empty list when entering the DixieDean Repo it just needs a force refresh. To do this simply bring up the context menu on the repository and click "force refresh". Now enter the repository again, it may still be empty - if it is just come back out and go back in again and it should re-populate (no need to redo the force refresh command again).