Well I see you got your answer, but that isn't really the best solution, rubymine already caches all the function names and index it to find it fast without doing a search, try this method instead, it should be faster
Navigate -> Symbol
You'll get a small popup search bar, as soon as you write cook
you should probably find your function in an auto complete, depending on your layout settings you'll find a nice shortcut to open it.
As a plus there's also
Navigate -> File
I never search for a file on the side bar to navigate, use this, it's always faster and will list all files with auto complete
The normal find function mentioned is used when you're trying to find a normal line of code, not a name of a function or a file.
Note that in both these popups you don't need to write the full name, for example to open the cookie_monster
function, you can search for it as this comon
( co
for cookie, and mon
for monster ) you'll find it fun and very fast.
As for the find usages
it's common usage for me is to find where a certain partial is called, or a certain function is used.