AI files don't require a codec. They are a vector format -- and they were definitely supported in earlier versions of the program.
It could be that they're no longer supported. Though, since AI is an Adobe product and most Adobe products can use other Adobe products natively, it would be a strange development.
Can anyone else confirm that the program will no longer take AI file natively?
If so, you can still use transparent GIFs or PNGs to the same effect.
Welcome to the forum.
I was surprised by the lack of native support for AI files in PrE, but this Adobe KB Article on Supported File Formats, no longer lists AI.
Now, as Steve mentions, AI files are Vector art, and for Video, they will need to be Rasterized at some point, since Video is ONLY Raster. I have experimented with Importing AI files into PrPro, and also using Photoshop to do the Rasterization, and found that at least through CS4, Photoshop did a much better job with the Rasterization process, than did PrPro.
If you have either Photoshop, or Photoshop Elements (think that it handles AI files, but could be wrong there too?), Place the AI files, and then Save_As PSD, Photoshop's native format, and one that PrE can handle nicely. My second choice would be PNG, with TGA farther down the list. I am just not a fan of JPEG, with its compression, so that would be about the last format, that I would choose.
Looking for the AI support, I did notice that PrE now supports Camera RAW Import, but that will be dependent on the exact flavor of Camera RAW, and having the appropriate ACR (Adobe Camera RAW) module installed. Quite often, Adobe updates the ACR module, to include newer Camera RAW formats, so if one has a new camera, and its RAW files do not Import, updating to the latest ACR should get that support - I learned something new, thanks to your thread.
Happy editing and good luck,