Navigate to your track in Soundcloud and copy the URL as it appears in your browser. Make sure that it's the URL for the actual track, and not for your Soundcloud profile:
Then, on IDMF, select the Advanced editor:
Paste the Soundcloud address into the editor:
With that URL still selected, click the Soundcloud icon:
That's all there is to it. If there's a problem, you can try disable the 'Automatically parse links in text' option further down the page.