The best place to put the ExcelPython add-in is in the Excel startup folder. All files placed in this folder are automatically opened by Excel on startup, so if you place ExcelPython there you do not need to manually open it each time you want to use it. The xlpython.xlam file must be placed in the XLSTART folder and the xlpython folder must be copied alongside it like so:
Unfortunately the folder's location varies depending on the version of Excel. Some candidates are
Excel 2013 current user: %APPDATA%\Roaming\Microsoft\Excel\XLSTART
Excel 2010 current user: %APPDATA%\Microsoft\Excel\XLSTART
Excel 2007 current user: %APPDATA%\Microsoft\Excel\XLSTART
Excel 2003 current user: %APPDATA%\Microsoft\Excel\XLSTART
Excel 2013 all users: %PROGRAMFILES%\Microsoft Office 15\root\Office15\XLSTART
Excel 2010 all users: %PROGRAMFILES%\Microsoft Office\Office14\XLSTART
Excel 2007 all users: %PROGRAMFILES%\Microsoft Office\Office12\XLSTART
Excel 2003 all users: %PROGRAMFILES%\Microsoft Office\Office11\XLSTART
Note also that %PROGRAMFILES% may need to be substituted with %PROGRAMFILES(x86)% for 32-bit Excel installed on a 64-bit machine.
If you are in doubt as to where the folder is located, you can also determine it by opening Excel, opening the VBA window (Alt+F11), switching to the Immediate Window (Ctrl+G) and typing
?Application.StartupPath
for the current user and
?Application.Path + "\XLSTART"
for all users.