I am using following open source action script component
Worked OK for me, though the namespace works out a little differently for me in that the control is bound to xmlns:controls="com.flextoolbox.controls.*", making my mxml tag
<controls:CheckBoxList id="test" />
You might insert a new checkboxlist above the current line and make sure it still comes up in the toolbox namespace. If not, just finish typing that new one and it will bind the namespace, then you can use that line instead of the old one.