It is simple. You join Spottt, upload your 125x125 card and add a piece of code to your site, above the fold. Once your blog is approved, you will be displaying cards from other blogs. They in turn, will be showing your card. You earn one credit when showing a Spottt and it takes two credits to display your Spottt. Spottt has put a throttle on the system to keep large sites from gobbling all the credits.
The site is less than a week old, so it remains to be seen how well the system works. You may find more information at the site or the Spottt Bloggg.
If you want information on Entrecard, please visit CoolAdzine for Marketers blog, where I am documenting my experiences with that program.