To open an existing file do one of the following:
Press Open button in Welcome dialog (there is also a button to open the most recently closed file - Open last)
from File menu, Open, File... command
Click icon in the Main Toolbar.
RationalPlan Multi Project have the possibility to connect to a server and open a version (the latest or an earlier one):
Press Latest version from server button in Welcome dialog
from File menu, Open, Latest version from server command (loads most recently saved version of the plan by an user connected to server)
from File menu, Open, Older version from server... command (loads an older version for viewing and creating "what if" scenarios in a "read-only mode"; to start from a previous version and save modifications open the server's administration interface and revert to that version)