I have made a SSIS package to create an XML file, which works fine in Visual Studio, but when I try to run in cmd prompt using dtexec /f "C:\\OakeyCreek.dtsx"
You are creating the package using SSIS 2005, but running it using DTEXEC from SSIS 2008 - you probably have both SSIS 2005 and 2008 installed, and DTEXEC 2008 appears first in the path.
To use DTEXEC 2005, specify full path, like "c:\program files\Microsoft Sql Server\90\dts\binn\dtexec.exe" /f Package.dtsx
In theory, DTEXEC 2008 should also be able to run the package (it should upgrade it on the fly to SSIS 2008 format), but there might be bugs and incompatibilities between the versions. Also it seems something is wrong with script task setup in SSIS 2008. Why would not you follow the instructions in the error message: logon as administrator and (using elevated command prompt) run -
"C:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE\VSTA.exe" /HostID SSIS_ScriptTask /setup