Currently, to use the multiselect box you need to know which values you're going to need and add them from the Custom Field screen. However, if you're in the middle of creating a Test Case it'd be good to be able to add them through the custom field and not have to navigate away from the Text Case. For example:
If you've setup the following Custom Field:
Name: Reference
Type: MultiSelect
Values: 1, 2 & 3
When creating a Test Case, if you needed value 4, you could just enter the value into the 'Reference' Field and it'd automatically add/create the value/option.