1. Download/Install Avisynth Version 2.5.8 if its not already installed. Version 2.6 will not work and the developers are not interested in 64 bit support as far as I know. If uninstalling your current version of avisynth, make sure you keep your plugins folder or else you will have to get all your 32 bit plugins again.
2. Download Avisynth 64 bit 8-29-10 Release.
Note: There are reports that there is a resizing bug in the 8-29-10 release, but I use it and have no problems with it. Maybe someone more informed can come and explain this cause I can't. If this worries you, you can download the 4-16-10 Release instead.
Run the installtion script (provided by Turbojet of the Doom9 forums) included in the archive as administrator
This DLL will not interact or interfere with any of the functionality of your 32 bit Avisynth installation
Avisynth 32-bit is located in C:\<Windows Root>\SysWOW64, this version installs to (as backwards as it may seem) C:\<Windows Root>\System32
If this does not work for you, Copy included DevIL.dll and avisynth.dll into your C:\Windows\System32 directory
Again, it's backwards, but all 64bit binaries are located in System32
4. This DLL will only work with 64 bit programs, and 64 bit codecs. Remember this so you don't run into conflicts later on. Download Virtualdub 64 bit and DgDecode 64 bit if you use Dgindex to index your sources. Place the DGdecode.dll in your avisynth/plugins64/ directory and just to make sure their are no problems between DGindex and your plugin, I would recommend replacing the DGdecode.dll in your DGindex folder with the one you just downloaded. If you don't, you may run into problems when loading your d2v files when using 64 bit avisynth.
5. Go here and download all the plugins you want/need/use and place them in your 64 bit plugin directory.
6. Confirm that everything works. Index a DVD with DGindex and create a simple script. Test it with VDub64. If it doesn't work make sure you followed all the steps above correctly before posting here for help.
7. Download and install Pwolf's wonderful AVS Import plugin for CS5 and thank him for it later. I can't help with problems regarding this plugin, post problems in this thread if you need help.
I encourage exploring the usefulness of setmtmode() that was included in the 64 bit installation you followed. If you are using a processor that has more than one core or supports hyper-threading you can see significant performance/speed gains when it comes to editing and rendering with AVS files. The speed boost from this is insane and could even help the person who just wants to convert/encode their sources to an editing format. Anyway just want to encourage it, its not necessary.
Feel free to post any info in regards to avisynth 64 here or if you have comments, concerns, or just in general need help, don't hesitate to post. Figured I'd get the only guide out of the way that I felt confident enough to do myself.