To use Tagalog, you first need to open a GPX tracklog by clicking the "Open tracklog..." button in the welcome screen. Once you have successfully opened a tracklog, Tagalog will display it on a map so you can use it to geotag photos from a source.
Tagalog currently supports tracklogs in the standard GPX format. If your tracking device does not let you directly access tracklogs in this format, you may need to use other software to download and/or convert your tracklogs to the GPX format.