It is not necessary to put the Factory 2D assets in content browser design catalog. The default user library for both 2D and 3D assets are at C:\Users\xxx\Documents\Factory Library(supposed you are using Win7 OS.). And you can set the your library by Inventor Projects Eidtor>-Library Node.
Well, you need to specify the library set in Inventor by seleting the corresponding inventor project in Options>-Factory Asset .
Finally, you need to show the Factory Asset browser by Factory Tab>-Tools>Asset Browser. You will see the 2D assets in there.