Airport Utility can't see the airport, nor are shares visible (Why is there no very extensive 'cookbook' on troubleshooting Airport Extremes?)

Question

For quite a while my MBP (10.5.8) cannot see the Airport Extreme 802.11n (AE) (2nd ed) anymore when I start Airport Utility (AU) (v5.5.3). My solution was to lower security settings of the MBP to 'accept all incoming connections', knowing that this was not a nice solution. Normally I had selected 'acces for specific services and programs' (the lowest option - my AE differs in language). In the past I denied access for requests that regularly came from mDNSresponder (etc), as I could not easily find out how necessary they were. I am not sure whether Bonjour is running; I try to stick to TCP/IP (without Netbios); the same might be the case with other obscure network services...
Then I added another network drive (WD Mybooklive) that works based on shares. The installation program worked fine, I was able to set up the drive for Timemachine backups and -in the proces of upgrading to Snow leopard (I always stay one OS version behind)- my MP3 collection could be read from another drive that was connected to the AE.
I am not sure what exactly lead to the loss, but now I can not see the shares anymore in the finder. Unless I select 'connect to server' (Cmd-K) and fill in afp://ip-adres. I had some trouble with the MBP-drive and Disk Utily could not fix it in multi-user mode. Applejack-auto in single user mode finished after some repairs (catalog file and catalog hierarchy errors), but I doubt that causes the trouble.
In all cases, internetting and accessing the connected drive was no problem.
The funny thing is, my Mac Mini (10.5.8) however can see the AE with Airport Utility (different version) fine. Somehow I get the idea that the networkstack  of my MBP has been messed up, but I don't know what it should be, nor how I could check it, or how to repair the stack. But maybe it is not the stack at all.
Right now, I've just reset the AE by holding the reset button for about 10 s. The name has returned to 'Base Station xx', which is visible in AU, but when I click on another button ('manual configuration' or 'continue'), first the message "Are you sure you want to connect to another wireless network?". Finally no connection appears and the error 'AU cannot find your Airport' shows up (while the Base Station is visible in the left column) - or does not show up when I refuse to switch to another network.
So, who can help me to reconnect to the Airport Extreme and/or repair the network stack?

Answer 1

WDS is a difficult setup for many users because it is so easy to make a mistake (while swearing that you are not).
Apple's instructions for WDS are here:
http://support.apple.com/kb/HT4262
I still think the instructions provided by Tesserax are the best available:
http://discussions.apple.com/thread.jspa?threadID=2422028&tstart=0
I can't add any more to this other than to say that long ago when I used WDS, I would swear that I had all the MAC addresses entered correctly, etc and would still have problems. So, I ask my wife to watch me and she immediately catches something like I entered a "l" (letter L) instead of a number "1", or a "O" (letter O) instead of a "0" (number zero).
Finally, there's always the possibility of device that is not holding settings...a defective device.
Hopefully, Tesserax will have some additional thoughts.