Enter Find String (Cmd+E on Mac) is really useful way to find occurrences of currently selected text across file or project.
However there seems to be a bug, which prevents Cmd+E from working if Find panel (Cmd+F) was not shown yet (since app launch).
Steps to reproduce:
1. Close qtcreator and open it again
2. Open some text file (whatever)
3. Select some word (i.e. via mouse) and hit Cmd+E (Edit > Find/Replace > Enter Find String)
4. Press Cmd+G (Edit > Find/Replace > Find Next)
5. It has NO EFFECT
6. Now open via Cmd+F find panel and close it via Esc
7. Repeat steps 2-4, once you reach step 4 Find Next works now!