The system is telling you that you don't have WRITE access to the root folder of your DNN installation. As per the DNN instructions, you need full write access to all the folders in DNN so it could lead to problems

DSLocalizator, when installing the hack, tries to 1) create a copy of the default.aspx.vb in the root folder as a backup. 2) Modifies the default.aspx.vb file by changing 1 line of code.

If no write acces if found, the error will appear.