tab key stop??

I'm new mac user and found that a lot of the mac dialogs don't have tab-stops or it could be that I was trying to use the PC-style tab-stopping (simply press the tab key).

I wonder if Mac forces user to use mouse to hit certain buttons...
or it's just a different set of key control...

Somebody help plz.

 

I'm using P4V (perforce)
When creating a new changelist, after inputing the detail, I can't move my focus to the OK button with keyboard ....

And also when copying files in Finder, when I try to replace the existing files, I can't tab-stop to the Replace button either...

 

No, just tested, with Safari, if I try to bookmark a page, for instance, I can't tab-stop to the bookmark-title edit control or the bookmark folder combo-box...

Interestingly, I can tab-stop to controls inside the web interface such as this forum...

Thanks a lot!
That did the trick!

BTW, how to mark this thread as SOLVED as in the Ubuntu forum?
From the UI setup, it seems that this forum uses a similar system with Ubuntu forum.