Well, you can download AVC and its documentation from here
http://web.ics.purdue.edu/~serio/vc/.
Now, in order to get connected to vatsim you need SBRelay only if you are using FS2004. It is not needed for FS2002. As a briefing you need to do the following:
1. Subscribe to VATSIM and get userid and password.
2. Download and install needed software:
- SBRelay (FS2004 only)
- Squawkbox
- Roger Wilco or AVC
- FSUIPC.dll (see comments below)
- Servinfo (not necessary but very useful)
3. Update Squawkbox server ip addresses. It is possible that the servers' list available for connecting S/B (Squawkbox) to VATSIM are obsolete. To solve this you either check the current IPs from VATSIM or extract them from Servinfo. To do this start Servinfo (you must be connected to the internet) and press the button labeled VATSIM (somewhere at the upper left corner) and wait until data are retrieved. Once data are downloaded from VATSIM locate the button labeled IP (it is either up or below the VATSIM button, I do not have Servinfo handy right now) and prss it. This will extract the new IP addresses to a "ipaddress.txt" file and the program will ask you where to save it. Choose to save it to the defaut S/B directory. Answer "Yes" when you are asked whether to replace the old file.
4. Start your FS and create a multiplayer session
5. Run a program called "FSchat14.exe" (or something like that) and it is in the S/B directory. Minimize the DOS window which will open.
6. Start the SBHost
7. Start the SB Main program. Choose for the SB to search for multiplayer sessions and when asked to provide an IP give 127.0.0.1 (your computer). Choose the multiplayer you have created before (SB will locate it)
8. From the SB right click on the dark area to open the menu and choose connect. In the meny opened choose a server to connect (field upper left in the connection window with pop up menu). If you are using Roger Wilco you can also provide a R/W server at the bottom left field in the same window (I have not used AVC).
If I have not missed anything you will be connected.
Now, there is a comment about FSUIPC. If you have a version prior v3.0 everything is ok as it is freeware (but in FS2004) you may have a few problems). Later versions (I think the latest one is v3.1.1) are shareware meaning they are operational but applications like S/B will not cooperate with them unless they are (the applications like S/B) registered to FSUIPC. There are ways to bypass the need to buy FSUIPC but as you understand they are illegal and I cannot tell anything more about. In any case buying FSUIPC implies a very low cost, you will are totally legal and you have all needed codes.
All the needed programs to get connected to VATSIM (including AVC and Servinfo) can be found in several internet places but you can find all together at the Hellenic VACC
http://avsim.com/greece/hvacc/Just select the english language area (there is also Greek but it may not be of much use for you) and then go to links-->downloads.
I hope the above helps. Ask back if you have more questions.