Hi kharchi,
Currently, you can only use and preview the free and open-source Typekit fonts in Muse. Adding additional fonts from Typekit requires some code and a live site to preview the fonts.
If you're comfortable with a little HTML/CSS, the Muse Insider wrote a post on how to insert the Typekit embed code from the Muse interface:
http://museinsider.com/how-to-add-typekit-fonts-to-your-muse-site.html
Or you can add the embed code to the Page Properties dialog within Muse, which will include it in the <head> of the page when you export your website.
However, we do see the value of being able to design in Muse with all the fonts in your Typekit library. This will require a deeper integration between Typekit & Muse. I'll give the team a nudge in this direction for you.
Let me know if you have any further questions and thanks for your patience.
Cheers,
Benjamin