Hi Paulo!
Firstly, you need to update Perian. Version 1.2.3 is what I use on an older Mac running Leopard, and I know they have made a newer version for Mountain Lion albeit possibly still in beta, but I have read in other threads that it works.
The main problem here is that you are using codecs that are incompatible with iDVD, which will not handle srt files nor most avi files, so they need conversion to something iDVD can use, such as for example Apple Intermediate Codec, the latter being what iMovie converts my avi files to before it can use them. Alternatively a Quicktime format.
But perhaps in this case iDVD is not the right application for such files. I have never used it, but Wondershare's DVD Creator says it can do this:
http://www.wondershare.com/dvd-burner/avi-to-idvd.html
which includes this information:
How can I convert a .avi video and .srt subtitles to a DVD? AVI is widely used to store DVD movies and can be usually downloaded online. There is also subtitle for the DVD movies. To put AVI and SRT (or ***, SSA) together, use Video Convert for Mac (mountain lion supported) recommended in solution 2. You can easily add subtitle to AVI using the editing feature. After imported the video, click the "pen" button to open editing windows, and go to Subtitle tab to load external subtitle file from your computer.
DVD Creator:
http://www.wondershare.com/pro/mac-dvd-creator.html
is not free, but there is a free trial version you could experiment with before spending $49!