Please see the following message on the VOSON status page (http://uberlink.com/service-status):

It is not possible to search for seed sites. You can find seeds sites manually via a search engine such as Google.

So this feature is currently broken in VOSON+NodeXL and VOSON web app, and we don't have a planned date to fix it.

It is easy to search for seeds via e.g. Google and then copy and paste them into the "add seeds" window.  Actually this is probably preferable for most users, since then you will have local search results.

Rob