In Vivado 2012.2, the IP Catalog is only available for RTL sourced projects. Vivado 2012.2 post-Synthesis or post-Place and Route projects do not have access to the IP Catalog from the Vivado GUI.
In Vivado 2012.3 and later, there is a "Standalone IP Catalog" option which can be selected by using the "Manage IP" option when the Vivado IDE is opened without a project.
Even though the IP Catalog can be viewed in the Vivado 2012.3 tools with a netlist project open, IP customization is not available. In order to customize an IP core, you need to open an RTL project or use the Manage IP option with no project open.