On the File menu, click Recent Projects
This lists your 20 most recent projects, so you can click on your project name.
Note that when BuildPlus starts, it opens the last project you had open. If you only have one project, it will always be opened automatically.
If the project is already open, the following dialog will appear, to prevent multiple builds of the same project at the same time. Once you click OK, BuildPlus will start and allow you to open a different project.
(Under certain circumstances, the project may not open. If this happens, contact GMCL for assistance.)