Are you saying a new window opens, or just another page is displayed?
If the former is the case, wait for the window to show up, switch to the window, and wait for the button to be present that you need to click on.
If the latter is the case, you simply need to wait for the button to appear on the new page. I use a wrapper routine for those cases that verifies the right page is loaded by checking the title, so it does the click and then waits for the expected title. If another title shows up that is an error that causes an exception to be thrown.
Mike
Mike