Normally the steps are:
1. Extract the W7-compatible drivers to a local folder.
2. Load the image.
3. From the Drivers screen, load the folder containing the drivers.
4. From the Apply screen, expand "Reapply tasks across editions" and check boot.wim (Windows Setup).
5. Apply changes.
6. Use Rufus or another tool to write NTLite's created ISO file to the USB drive. NTLite will only create an ISO file for you, it doesn't handle the step of converting it to an USB filesystem.
For next time, when you mount install.wim, you can integrate them at the same time in boot.wim (Windows Setup) with the option Reapply tasks across editions (Apply page) and check boot.wim (Windows Setup)
I never use ISO