First download and install OmniGraffle. You won't need to buy it to try it out. You may not even need to buy it at all if your stage plots are simple, since it lets you add up to 20 objects before you have to pay. After you've downloaded Omnigraffle and tried it out a bit, download my stencils, which include an amp stencil, an instrument stencil, and an general purpose stage plot stencil.
After downloading it, unzip the file, and copy the stencils into ~/Library/Application Support/OmniGraffle/Stencils/. Next time you start OmniGraffle, it will see the new stencils and you can grab the images and lay them out on a document. When you're done, save the document and use the export feature to save as a jpeg, or pdf. I threw some instruments together and came up with this in no time flat:
(click image to enlarge)
Beautiful it a'int, but clear it is, especially considering all the stuff on stage. For details on what should really go into your stage plot, remember to see my blog posting on that subject, but you get the idea. If you have an instrument not covered (I only did the basics), you can either use a box and label it or find a picture of it online.