To edit with your avisynth files in Premiere, you will need to download the latest Premiere Import Plugin. If you have downloaded and installed the AMVapp then you already have it installed. If you have to install it manually then copy it to your Premiere 'Plug-Ins' folder.
One you have this installed, it will allow you to open AVISynth (.avs) files in Premiere.
Note: this method isn't particularly quick at the moment. Seeking and rendering will be slower than you are probably used to. There are things that can help, but only a little. What I recommend is that you get yourself a fast codec (like the PICVideo MJPEG Codec) and select this as the codec in your "Project Settings". This will be the codec used for previews. You can always use a different codec later for exporting but this one is great for fast renders - especially when set to a low quality level. So, if you need to see if something has timed correctly, mjpeg codecs are great for that as they are very quick to decompress when set at low quality.
However, if you find that it's too difficult editing with footage this slowly, then please use method 3.
Note also that this method of using footage is even slower if you use IVTC in your avisynth script to remove interlacing - by this I mean, MUCH slower. However, you can save yourself a little processing time by using Telecide(post=false) instead of just Telecide() It wont give you that much of an increase in speed, but it's better than nothing. If, on the other hand, your footage is natively non-interlaced (such as a progressive dvd) then you won't need this step and won't have the slowdown.
Again, if you find that it's too difficult editing with footage this slowly, then please use method 3.
A few practical points to note...
If you slow down clips in Premiere to very very slow speeds then it will crash avisynth when you try to export or reload your project. Do not do this. Either make a still and use that or if it crashes when applying lots of filters, try and pre-render the section with a lossless codec.
Also, if you load lots of avisynth files into your project it may become overloaded as every time premiere opens an avisynth file, it eats up a large amount of memory. You'll definitely want to set your virtual memory (pagefile) to very very very large.
Now that you're done, you should go to the section on setting up your Premiere project.