|re: I've created a macro and saved it ...|
You can't actually save a macro, at least not directly. What you can do is save the workbook which holds the module that contains the macro.
re: ...in the XLSTART menu
XLSTART is not a menu, it's a folder.
That said, any workbook stored in the XLSTART folder will open when you start Excel.
If you want to have macros available whenever Excel is open, the typical practice is to store the macros in a file called personal.xls. You then hide this workbook and save it in the XLSTART folder.
Whenever you start a new session of Excel, it should open personal.xls hidden and then open a blank workbook. It will also open personal.xls hidden whenever you open a saved file.
Whenever personal.xls is open, all of the macros stored in it are available in every open workbook.
To edit macros in personal.xls, go in through the VBA editor, not through Tools...Macros...Edit. That way you won't have to unhide the workbook to edit the macros.
Another tip: You can add icons for your frequently used macros to your toolbar so that they will be one click away whenever Excel is open. I have about a dozen macro icons on my toolbar and another half dozen in the right-click mouse menu.
You can also add an icon for the VBA editor so that you can open it with one click.
If you need help setting up those icons, come on back and I list the steps.