There are two possible causes of this issue.
1. Missing required Product: This is the most common cause of a missing OBJ. Please check the Required Products section, on the right side of the item's Product Page, to ensure that you have what is necessary to use the product.
2. Corrupted RSR file (Legacy, Poser 4 only): The solution is to delete the rsr file associated with the obj. It is a file made by poser everytime you open the model. Occasionally it creates a bad copy and needs to have it erased. It is located in the same folder as obj file. Once you delete the file Poser will create a new file and the model should open normally. Example: for the error "Cannot find the file blMilWom.obj." Run a search on your computer for the file blMilWom.rsr. Delete that file. Do not delete the file blMilWom.obj. If you accidently delete the wrong file you can re-install the product to replace it.