To import an existing workspace, click on the menu "File -> Workspace" to open the workspace management dialog and click on the "Import..." button.
Then a sub-menu pops up and you must select if you want to :
·import a workspace from a folder
·import a workspace from a ZIP file
·import components from a folder into the current workspace
·import components from a ZIP file into the current workspace
If you just want to use an existing workspace without copying data you have to switch workspace.