I’m using Anki for iPad with Dropbox too, and had similar problems. I asked on their forum and was simply referred to the docs, which to me are a little confusing and not really well organized.

I know it’s a pain, but go here and read carefully. Particularly in the Limitations section, you’ll see that Anki doesn’t deal with spaces in file names, hence the % you see. So you’ll have to rename those media files and remove the spaces or other characters that may not be allowed.

Then look in the Problems section and follow carefully the steps needed to add new media. Hopefully that will take care of your problem!