In an OOB instance the cmdb_ci list, will have that field is greyed out so you cannot enter manual search terms.  

 

Because of the unique qualities of the sys_class_name field in the CMDB, this field is locked down.  The class field is the mechanism that links the base CMDB table to its' extended tables and it was determined that this field needs to be locked to prevent changing its' value.  A modification to this value will orphan records.

 

The only option here is to search on Class from the standard search tool at the top of the list.