Have you done the xdelta against the original portal game to get the Media.zip and decompressed that into the app directory? Because if you have I've really screwed up somewhere!
EDIT : Fixed, the ux0:app folder you mention on the install instruction is incomplete, it mention folder "APTR1007" but it is actually "APTR10007".
Maybe if i installed the vpk first it would have been already created and i would have seen that it was the right folder
I also have the exact same screen. Everything was installed as described, extracted the Media.zip from Steam files, copied to ux0:app/APTR1007/Media
Then copied portal_pak_dir.vpk to ux0:data/AptrRecon/ (i have the french version of the game, i guess i have to do it also ?)
then i installed the vpk using Vitashell. I launched the game, and i have this screen as dexter2939