You can always download them and open the files. Something is awry with the association and command line or where your temp files go; it's hard to say without seeing your computer. Right click on the the tab file links and choose download and put them somewhere on your computer, then open them from there. You won't need to be online to play. BTW, as far as simple songs go, you can certainly use your guitar knowledge and songbook and just look up the same chords for mando. Open C, G and D are all 2 finger chords and you will get plenty of mileage.
"First you master your instrument, then you master the music, then you forget about all that ... and just play"
Charlie "Bird" Parker