I don't have a download link for the font but I remember when I once needed this font I went to Google and just searched for it. I then went to
http://www.linotype.com/46494/myriadsemibold-font.html and clicked on "Create Sample". Although you can't save this font on your computer and use it in applications, you can type text into the sample font creater on Linotype, copy the gif into Photoshop or Illustrator or whatever and then put it in a banner etc. Helped me a lot for creating very nice looking fonts in my banners/presentations and stuff. Good luck. Then again if you're rich and don't mind forking out 35 dollars for the font might as well do that.