This is not possible in this beta release. Adobe recently bought TypeKit so it would not be a surprise if this works its way into a future release.
thanks for the fast answer!!!
but what a pity. Is it possible to use the @font-face command somehow?
Defining a normal textfield with arial or something and tell that textfield to use @font-face?
Grab fonts and HTML/CSS from http://www.fontsquirrel.com and then manually edit the HTML produced by Muse... or use the 'arbitrary HTML insert' function within Muse. Neither method is pretty if you want to adhere to the no-code approach of Muse.
I'm sure you know how easy it is to add fonts with plain old HTML/CSS. But it takes Adobe much longer to engineer a way to profit from this free task.