What computer operating system is your Premiere Elements 12/12.1 running on?
You cannot import one Premiere Elements (any version) project file (project.prel) into another.
You can transfer from project to project by
a. exporting the content from project 1 to project 2 by exporting the Timeline content from project 1 to a file saved to the computer hard drive in a format compatible with the project 2 and then importing the saved file into project 2 with project 2's Add Media/Files and Folders.
b. if your Premiere Elements 12/12.1 is running on Windows, then you can consider ClipMate (Windows Only) in conjunction with Premiere Elements for a copy/paste insert between two Premiere Elements projects.
Any questions or need clarification, please let us know.