How do you add games to EZF advance with DS?

I've seen threads with methods to add games to EZF advance using a ds flashcart and a ds, however, I can't get mine to work? I'm using the DS x-treme, and I'm following the instructions in this post.* I've tried pressing start and select while booting up slot 2, as well as normally booting. I don't want to flash over the cartridge, and my gba is in repairs currently, so I can't add games normally.

Am I just missing a step? Is the Ds-xtreme not compatible with the guide?

Thank you in...

How do you add games to EZF advance with DS?