The way I hear it (corrections are more than welcome) Page is using a technique called displacment. This simply means that the same phrase is beginging at different points of the bar. As far as I can tell the black dog riff is being played a 16th note later each time it is repeated which is what is throwing you off. It's very difficult to get the hang of and all I can advise that you do is to play it slowly taking careful note of which notes fall on down beats which will hopefully help you get the feel of it.

As for how to get timing from tab- unfortunately that is one of the disadvatages of tab and you have to try and figure it out yourself (as far as I can tell!). Most professional tabs will also have the musical notation though.

Hope that is of some use.

AndyP