Excel 32-bit to 64-bit

Hi,
The accounting department uses some older Excel sheets that declare functions like this:

Declare Function GetUserName Lib "advapi32.dll" Alias "GetUserNameA" (ByVal lpBuffer As String, nSize As Long) As Long


After upgrading one of the users to Office 2013 Professional x64 they get the following error:

Compile error:
The code in this project must be updated for use on 64-bit systems.
Please review and update Declare statements and then mark them with the PtrSafe attribute.

I'm not too sure what that means. What do I need to do to get this working?

Thanks,
Stan