Hi Nico, probably the easiest way is to use curve-to-curve flow. Create horizontal text, draw a line under your text and a circle off to the side. Then select your text (only the text, not the line under it or the circle) and run Transform > Deform > Flow. Then pick the line as the base curve and the circle as the target curve.

See here for some examples of curve-to-curve flow:
http://moi3d.com/3.0/docs/moi_command_reference8.htm#flow

Also having the line be the same length as the circle circumference can be good to have better control of how it's going to be spaced on the circle. To do that you can draw your circle first and then create a line of the proper length using the UnwrapCurve plug-in from here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=5136.1

- Michael