Poser 4 and 5 don't do omnidirectional lights (don't know about P6). In P4 and 5, omni lights can be facked by using a cluster of spotlights, a psudo-omni. All the paramiters of the cluster can be controled from one set of dials using ERC. My 'P5-Omni-12 (6 Omni lights)' is such a psudo omni light, it's available here in the Free Stuff (not tested in P6). There is also a P4 version.