Are you sure that the liveries you are trying to add to the 757 are able to be added using the ACE? Or are they supposed to be added manually. Many of the repainters do not make them so they can be add using ACE.

Within the Zip file, there must be:
a description.txt
a texture.cfg
a thumnail.jpg

If either of the first two are missing, then they might need to be added manually. Instructions should be in a "readme.txt" file (or something similar to that) telling you if it can b added via the ACE program, or have to be done manually.

I do mine manually regardless of whether they can be added using ACE or not.