Does your content list in Content Manager? Does it have an error flag? If so, what are the errors?
Does it show in the surveyor list? Is the listing white or red? Have you checked that the item is not either far too small, or far too large (scaling errors happen to all of us, especially if you are converting your models between programs)? Is your model on the origin? Sometimes it can appear a long way away if it's not centred.
Paul