To convert emitted nParticles, either emit some nParticles into the scene and select them, or select the nParticle object from the Outliner.
After your nParticle object is converted to a polygon object, Maya no longer displays the particles in the scene view. This reduces simulation time and makes it easier to see how your Output Mesh attribute adjustments affect the mesh.
To make the nParticle object visible in the workspace, in the Object Display section of the nParticleShape node Attribute Editor, turn off Intermediate Object.