How can I create a Windows 7 USB from Mac OS to work on a PC?
Usually the person asking the question does not give enough information for a answer to be generated. So, I will try anyway.

Does, the PC have a working bootable DVD drive? If so, just use the OS X Disk Utility application to burn the iso to a DVD.

If your PC can boot from a flash drive, then you have two options. First, use some machine running windows to transfer the iso to the flash drive. Or, install windows 7 to a bootcamp partition on your Mac. Do not activate windows. You can use it for 30 days. Make the iso to flash drive transfer and install on the PC. Finally, remove the bootcamp partition. The last step is fairly easy, if your Mac is not using Core Storage. 

The Microsoft software to transfer to the flash drive can be found at Windows USB/DVD Download Tool. The tool does not download software. Rather, the tool assumes you already have the iso and need to either burn a DVD or transfer to a flash drive.

If you edit and refine your question, I can update my answer to better fit your problem.