I've just got the trial and found this a real pain. I suspect once you know all the ins and outs its a gem, but for now...
Ok, here's what you do to get the basics. First up, you have to configure it to work with flickr. Give it usernames and passwords etc. I presume you got that far, if not lemme know.
Ok, so, now you want to upload. Lets presume you've been using the develop module. Switch to the library module and select grid view. From the filmstrip at the bottom choose the image/images that you want to upload. Now, hold down the left mouse button and drag them accross to the photostream. Next, click on the photostream. It'll give you the option to publish. Click publish, and the images appear on your photostream.
Haven't yet figured how to add to sets etc other than manually after upload.