I'll outline how I make a installer for an Excel Add-in - this might give you ideas for your own problem:
Let's say the name of the add-in file is 'MyAddin.xla' and the installer is called 'MyAddinInstall.xls'
In the 'ThisWorkbook' module of MyAddinInstall, write the following code:
Private Sub Workbook_Open()
FileToInstall = ThisWorkbook.Path + "\MyAddin.xla"
Set MyAddin = AddIns.Add(FileName:=FileToInstall, CopyFile:=True)
MyAddin.Installed = True
MsgBox ("The MyAddin Add-in is now installed." + Chr(10) + "Excel will now close." + Chr(10) + "The Add-in will now auto-load every time you start Excel." + Chr(10) + "Uninstall with Tools>Addins & deselect the MyAddin check-box."

Application.Quit
End Sub
For this to work, both the installer AND the Addin must e in the same directory. Note that the addin will be copied to the addins folder only when the installer is on a removable medium, say a floppy. If both the files are on your hard drive, the add-in will be installed but will NOT be copied to the add-ins directory - it'll be accessed straight from its current location.
Hope this helps.