The multi-select list box can only be used with many to many relationships via a linking object. You can create new linking objects (see the section "Linking unlinked business objects" in the LDSDDesigner.pdf manual) however there is a limitation at the moment on which objects you can create links between. I've logged an enhancement request to have this lifted, you can vote on it here: Allow linking objects to be created between any type of object.
Please vote for Stu's enhancement request. Currently the functionality only allows you to create a linking object between objects that appear in the Administration bit. So you can only create many to many relationships between these objects - users, groups, roles, categories, reference lists and ordered lists. You can not create many to many between any other custom objects or even IPC's or configuration objects.
I would like to vote for this ER but cannot see the link to the ER properly (showing 'not authorized').
Is it really something that has to be voted for? I would define it to be a not fully implemented feature. I was enthusiastic about the feature until I saw that limitations. The use of it is very limited as it works today.
Can you try again to see if you are able to access the ER?
Yes, I wanted to use this as well, but upon reading this I realize I will not be able to use it as I had originally intended.
I wanted to create a List of 'categories' and populate it onto a noticeboard item to allow me to select multiple categories (or groups) to receive an RSS feed (setup with query)
I see that this is a very old ER on a very obvious thing that needs to be implemented. Was this ever sorted out?