Be sure you clicked the button to "Show Community Tables" because that adds a bunch more tables.
If still no luck, you might be able to use one of the "generic" Data Tables under the "Data" Category.
Click on the "Desc" icon to see an example of the query format for each Data Table.

For example the xml Data Table says:

select * from xml where url='http://rss.news.yahoo.com/rss/topstories'

Enter in the url of the web service you are interested in and see what happens :PAs a last resort you can always write your own YQL Open Data Table.