Thank you for the clarification.
I may have misunderstood your original question. If you are asking if your AirPort Express can "extend" or "boost" the wireless signal from your Netgear router, the answer is that it is not compatible for this purpose.
The AirPort Express will only "extend" or "boost" a wireless network if it has been created by another Apple router.
Your Express can "Join" the wireless network that your Netgear router is broadcasting and you will be able to use it to stream iTunes as you have done before.
Perhaps the Apple person was suggesting that you reset the device to erase its current settings and try to reconfigure it again. If the Express is located 25-30 feet from your Netgear, it should receive a strong enough wireless signal to operate correctly if there are not too many walls or other obstructions between the Netgear and the AirPort Express. Cordless phones can also cause interference issues if they are located near a wireless device.
It would have been helpful if the genius had provided you with more information about how to do this. Perhaps he did and you did not mention this. I have attached the
Setup Guide for your reference.
To reset the device, make sure it is powered up. Hold in the reset button with an unfolded paperclip or similar object until the amber light begins to blink quickly. Hold it another few seconds and release the reset button.
Now you are ready to reconfigure the AirPort Express using AirPort Utility. This is located as follows: Hard Drive > Applications > Utilities > AirPort Utility. Follow the guided setup to configure your AirPort Express to "join" an existing wireless network. You will choose the network that the Netgear is broadcasting and enter the password normally required to join your network.
If the AirPort Express still does not perform reliably, then I would suspect that you either have a defective device or the Express is located where it is not receiving a strong enough wireless signal. You can test the signal strength if you have a laptop handy by trying to log onto the internet at the same general location as the AirPort Express. If your computer works well at that location, that is an indication that you have adequate signal strength for the AirPort Express to function correctly.
Please post back on your findings.