In reverse order: The circle tool is hidden behind the rectangle tool. Click and hold on the rectangle tool, and a few more shapes pop up, including elipse. Use this with the shift key held down to create a circle.
Transparancy is a little misleading though. With PNG files, and in Fireworks itself, you can have lots of different strengths of transparency. An item can be totally transparent, semi-transparent, totally opaque, or it can fade between all three states.
Although some browsers support PNG files, the only file format that you should use for transparency is GIF, for compatability reasons. Unfortunately, this only supports two levels of transparency: either a pixel is transparent, or it's not. This means that you won't get the same softness as you see in Fireworks.
To get this to work, in Fireworks you must choose GIF as your output format. Look in the Optimise palette for your options. One is transparency, and it is usually set to 'No Transparency'. Change this to 'Index Transparency' and click the first eyedropper at the bottom of this palette (Add color to transparency). Clicking anywhere in the image will make that color transparent.
If you want smoother transparency, you must cheat a little. Simply create a background in Fireworks that matches the background in your web page. When you export, it won't be transparent really, but it will appear that way to people viewing the site. Of course, backgrounds that are one solid color are easier to get to work than, say, a patterned background.
hth
