Search Engines are obtained from the referral list only if they include certain parameters that contain keywords. With Urchin 5, the keyword parameter list is configurable on a 'per profile' basis. Look at the Referral Keywords Match entry under the 'reporting' tab of any profile in Urchin 5. This list of parameters defines which referrals are considered search engines.
For instance, if a referral contains kw=urchin somewhere in the query string, then this referral is considered a search engine. Using this information, you must determine which parameter holds the keywords for the search engines you are interested in (and currently missing). For instance, example-search.com uses "searchfor" as the parameter. Adding "searchfor" into the keywords match list, deleting and reprocessing the data would cause example-search.com to show up as a search engine.