Okay.. let me summarize it here:-
-You created a macro (vba) and you saved the workbook as xla.
-now you wish to use this macro (or say the workbook containing macro) as an add-in in some other workbook.
-you opened your other workbook and browsed xla file and installed it. Now it should appear as a button (menu / ribbon bar) if you designed it that way or whatever customization you done to use that. If you wish you can search internet on how to transfer xla on menu / ribbon bar

You cannot see :-
the macro if your bring macro list box (alt + f8), see below what Microsoft is saying:-
"Excel Add-in (XLA) is really an Excel Workbook (*.xls) that does not contain any worksheets. The only part of the file that is used by Excel is the Visual Basic for Applications (VBA) project sub-storage, which contains the add-in (VBA) code.
http://support.microsoft.com/kb/268016"

You can SEE:-
if you press Alt + F11 (VB Editor), you should find your xla workbook listed on left side.

Below method can be used to convert an xla back to xlsx / xls :-
http://support.microsoft.com/kb/211561

hope this helps

Regards,
DILIPandey
<click on below * if this helps>