If you have the lastest version you should find it in Settings->Antivirus->Exeptions->File Path…up to translations mistakes since I have italian language for my version…
Anyway another option could be to recover your original exploer.exe. If you used W7SBC you find it in the Windows folder named as explorer_backup_w7sbc.exe. But you should be able to recover it directly via W7SBC…

EDIT: sorry if your question was how to add a single file instead of the whole folder, then just write C:\Windows\explorer.exe