all you need to do to slow it down is change the tempo. by default the button to modify the tempo is the 2nd one from the left on the bottom toolbar. it looks like a solid music note with an = next to it. the first time you click it the tempo marker in the tab goes away, clicking it again opens the tempo properties and you can set the beats per minute in that window. the keyboard hotkey for the tempo marker is the letter O.
and chiva was right. open D requires tuning the A string down to G also (i was kind of drunk last time i checked the forums last night so i didn't bother posting

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