If you wish to insert a project between two other projects at the same level, you should select the project BELOW which you want to insert a project, and
use the menu option Edit → Insert Project or
click the Insert Project button in the toolbar
Quick-Add: When editing values in the Summary column, hitting the ENTER key causes a new project to be inserted below the project being edited, and starts editing the new project's summary. Repeating this process allows the user to quickly enter a list of project summaries. To end the process, instead of hitting the ENTER key after entering the summary, use the TAB key, or click on another row.