I am trying to create dialogs looking something like these (from another piece of software) 
After much searching the best I can come up with is this:
CreateDialog[
Grid[{
{"Would you like to look for a text file containing dissociation constants?"," "},
{Item[ChoiceButtons[{"Yes","No"}, {DialogReturn["Yes"]; DialogReturn["No"]}],Alignment -> Right],SpanFromLeft}}, Alignment -> Left]]
which shows as

My only problem is in determining which button has been pressed because DialogReturn["Yes"] doesn't work.
I never guessed it would be so difficult to get a simple Yes/No dialog!