How to get Recorded Mixes that I made?

First, find the file -- there are two ways:

1) (Quicker method) From the Recordings branch of the tree (bottom left), select the recording you want, right-click and select "Show in Finder" (Mac) or "Show in Explorer" (PC).

2) Find out where you've saved the files by opening the preferences, then find that location in your files:

Then just upload the file to SoundCloud.