Oh I see. You're using the vox-l simulator. I think the particle blocks are not a block that's currently available in vox-l.
Either particles are too advanced for the vox-l simulator to render yet, or they were removed to conserve data space -- the vox-l simulator uses alot of data when running in the browser, so we try to include only the basic blocks.
They'll probably be added to the vox-l simulator after Development moves the advanced level badge tutorials from the old simulator into the vox-l simulator. Not sure when that will happen though.