There is basically no difference between older versions and Poser 11, content installation is just about the same. However, you didn't elaborate how your Poser app was installed and where did you install your content that didn't take. Unfortunately, although free content passes the minimum requirements to be available in Rendo, the compressed folder hierarchy may not conform to the actual library structure that exists in Poser. A properly constructed compressed file should contain a primary folder entitled 'runtime'. If such a folder doesn't exist, then attempting to decompress the file directly into Poser becomes a sure disaster unless you are familiar with the Library folder structure. But to help you with the problem, you should answer the two questions proposed to you.

To answer your second part, no, not all program can share content with each other. DAZ 3D and Poser can share content but there has been a widening divergence in recent years. There are importers and exporters but given the prohibitive cost of some other apps would you spend serious money to work with a lower end program?