To save a worksheet that you have created, select the Save item from the File menu. This will pop up a file selection dialog. Type in a name for the worksheet at the end of the string in the Selection subwindow and click the Save button. By convention, names of Maple worksheets end in .ms, so if you leave it off when you type in the name, Maple adds it for you. The name of your worksheet is shown as part of the title of your Maple window.
After the first time you have saved a worksheet, selecting Save saves your changes to the same file, without the dialog box appearing.
These instructions don't apply if you have loaded a worksheet that belongs to somebody else. For example the projectile.ms worksheet you loaded is owned by one of the Math faculty. If you tried to save it, you would be trying to modify a file that you don't own and this isn't allowed. To save your own copy of a worksheet you have loaded, use the Save As... command instead of Save. Once you've selected Save As... from the File menu, the procedure is the same as described above for the Save command.