The stencils are actually pretty easy to use.
To start, open the stencil palette by clicking the icon. Scroll down the list in the window and choose what you want. It should then appear pinned to the canvas. You can then move, rotate, or scale the stencil using the same move, rotate, and zoom tools you use on the actual canvas. pick one of the tools and move the pointer over the stencil make the pointer shrink, then you click and drag it around. If you want to rotate the stencil around a point that isn't in the centre just click and drag on the pin and move that to a new position, the rotation tool will rotate the stencil around that point. Then pick a paint brush and paint over it.
You can load more stencils onto the canvas and place them on top of one another to combine stencil effects like textures and gradients. If you want to get rid of a stencil just right click on it and select remove or hide etc. The ruler stencils work the same way but you also have the option to lengthen them by grabbing the green handle at the ends and pulling them out. If you wanted to create perspective lines for example, you just place a ruler on the page, move the rotation point and draw along the edge, then repeat. Or, you could add several rulers onto the page all starting from different vanishing points and just hide them when you want to work on the page.
As for tutorials, I don't really know of any other than those that have appeared in the magazine. Maybe there are some on ambient designs website but I haven't been over there for a while.
[edit] I also forgot to mention you can create your own stencils from greyscale png files and load them in through the menu in the stencil window.
