How can i show only one button with "Ask for confirmation" action in Automator.

That is just the way that particular action is written - even on Lion it winds up crashing half the time if you remove the text from one of the buttons. You can look for a third party action, or use a Run AppleScript action to put up the dialog instead, for example:

onrun {input, parameters}

display alert"alert"message"your message"

returninput

endrun

Sure - just add that parameter (there are a few others) to the command:

display alert"alert"message"your message"default button"whatever"

Automator's scripting dictionary has been pretty much gutted in Lion, but the functionality is still in there - you can look for the command in Snow Leopard's dictionary, though (is is a bit different than the reguar AppleScript alert).