Hi Nic,
Being unable to choose a module on your StudentHome page could be for a number of reasons. One possible one is that the module may only have an October start date, and the deadline to register was 17 September.
If you would like to discuss your options, or to register over the phone, you can call us on 0300 303 5303, Monday to Friday 8:00-17:30. If you wanted to explore if a late registration is possible, I suggest calling in as soon as possible.
Available contact methods and opening hours can be found on the ‘Contact Us’ section of The Open University website, or by visiting: http://www.open.ac.uk/contact/