Steps to reproduce:
1) In any Test Studio project, create a Web Test and record the following two steps:
	i) Navigate to: http://www.asp.net/ajaxLibrary/AjaxControlToolkitSampleSite/ListSearch/ListSearch.aspx
	ii) Select any option from the first ListBox sample (eg. 'Bravo')
2) Go back to Test View and highlight the second step created above (eg. 'Select by value...')
3) Under the step properties, open up the SelectedItems drop-down

Expected result:
Items can be added and removed through the UI, this property allows the step to select multiple items consecutively.

Actual result:
Add button is always disabled. Additionally, on occasion, the Remove button does nothing. Refer to attached images for example.