The soundcloud tag is for linking to a specific soundcloud track.
[soundcloud]http://soundcloud.com/artist/trackname[/soundcloud] - just copy/paste your track link between the tags.
The cloudset tag is for linking to a soundcloud "set" or playlist
[cloudset]http://soundcloud.com/artist/sets/setname[/cloudset] - just copy/paste your set link between the tags.
You can automatically insert the SOUNDCLOUD and CLOUDSET codes using buttons on the "Post" screen.
The SOUNDCLOUD button is orange and the CLOUDSET button is green, and they appear right below the BOLD and ITALIC buttons. These will insert opening and closing tags for the function in question, and you just need to insert your URL (see instructions above) between these tags.