1: Make sure to use the .sim file (which you can find in My Documents\Electronic Arts\The Sims 3\SavedSims) for your pet, rather than a .sims3pack or .package.

2: In addition to the required .sim file, you have also uploaded your pet as a .sims3pack. Mod The Sims only accepts pets as .sim files, please remove the .sims3pack.

5: You've uploaded your pet in a .sim file, huzzah!