excel menu item

I have an add-in. It is for a device that takes readings. I should then be
able to open excel and import the data from this device. I am running excel
xp on windows xp professional. I go to tools, add-ins, browse and select the
..xla file and this creates a new menu group. The device will not work
correctly. When I close excel, and then reopen it the menu item is gone and
I have to add it again. I am thinking this might have something to do with
it because the device worked on other machines. I can't figure out why the
menu item keeps unloading everytime I close excel. I have also searched for
the .xlb file and it will not come up in the search. Any help will be
greatly appreciated.