Do you mean you added a tab or a tab page? Anyway the solution is the same.
Just set the AutoDeclaration property Yes for the tab (page), and call myTabPage.visible(true) or myTabPage.visible(false) based on your condition.
I don't know your condition, but usually these kinds of changes to the form appearance should be made in the init method of the form, after the super() call.
Many standard forms have a method called updateDesign() which is called from the init method.