Well, I'm assuming you are downloading them from www.powertabs.net. The site will tell you what instruments are in the tab, so if you have one that has bass in it, before you play it, click the blue bass clef.
Just download the tab, open it in Powertab. Be sure to select "Bass score" in the View-menu, or hit F3.
In case you haven't figured it out yet, powertabs usually have more than one instrument in them, that might be why you're confused.