I'm also trying to use this search/query
It certainly says in the documentation that querying by Lat/Long box should be possible.
Search searches data on all airborne aircraft to find ones matching the search query. Query parameters include a latitude/longitude box, aircraft ident with wildcards, type with wildcards, prefix, suffix, origin airport, destination airport, origin or destination airport, groundspeed, and altitude. It takes search terms in a single string comprising "-key value" pairs and returns an array of flight structures."
I get the same error about the need for an even number of elements.
I'm going to play around with some query strings to try and guess the proper format. But some documentation help from a staff member would be helpful also.