I found this thread after having a similar problem installing the Tropico 3 - Absolute Power expansion that I bought and downloaded from Amazon. I noticed that, during avatar customization, some selections would cause the avatar not to show up, or to show up with a giant box with an F on it. When I reinstalled the game and expansion, I noticed the "copy failed" messages when I installed the expansion. I took Polahrki's suggestion of copying the expansion files over manually, and that seems to have worked.

Not sure what's preventing the installer from copying the files; I don't use any antivirus software and I have things like Windows Defender disabled entirely.

Incidentally, I did notice that the expansion pack contains a file named MEMASSET.hpk, while the base installation contains a file named memAssets.hpk in the same folder. I hope MEMASSET.hpk isn't intended to replace memAssets.hpk.