i'm struggling with this also...i've read the walkthrough posted on the following link, but is does not help you find the 5th bomb..have you managed to find it yet
I figured out how to find the bomb. Make sure that there is not just another room below you on the 1st floor that will help. There is also a secret room that you see when you first break into the building. Just keep looking for an odd section of wall in that area.