It depends on the class you want to find is an actual model element, or just a “stub” created with the reverse source on demand option (a feature of Instant Reverse).
If it is an actual model element, you can find it via the advanced find feature:
If it is just a stub, currently VP cannot find model element stub created with Instant Reverse on demand option. So you either reverse the interested portion of stubs into actual model elements, or you can expand the tree nodes in the class respository, and type the name of the class to find. The class repository tree, as well as other trees in major panes (e.g. model explorer), offer basic searching facilities by typing, similar to those found in file manager. It selects the first occurrence of the node starts with the typed text, you can press the up/down key to navigate through the search results.