Goto C:\Program Files (x86)\ejabberd-2.1.8\conf (in my case) folder & open ejabberd.cfg file using Notepad++ (it is easy to edit using it).
In the file do the following changes:
%% Put this in the section ACCESS RULES
{access, register_from, [{allow, admin}]}.
%% Change mod_register so it contains the new access rule:
{mod_register, [
{access_from, register_from},
...
] ...