That widget was last released for Mendix version 2.5, so its pretty old (back in 2010). You could work on upgrading it to 7, but my guess is that would be a lot of work.
Also, there a number of other options in the app store that have more recent versions. One that I use for reference set selectors often is the Label Selector. I have not tested it on 7 yet, but It works well up to 6.10.3, so you shouldn't have too many issues with it in 7.
Hope that helps,
Try the AutoComplete widget from the app store. It should accomplish the same thing and is at least compatible with Mendix 6 if not 7 already.