WEP and WPA are different, so I will assume you are asking about WEP. (Ask a separate question if you want to know about WPA.)
Wireshark should be able to decrypt WEP traffic if you have the cryptographic WEP key. For instance, here are instructions for decrypting WEP and WPA traffic.
So all that is needed is a way to find the WEP key. There are several ways to do it:
If you are connected to the WEP network, then your laptop must have the WEP key. So all you need to do is extract it. The way to do this will be OS-dependent, so if this is what you want to know, ask a separate question about how retrieve the WEP key for a wireless network you are connected to, and specify the OS. (e.g., on Linux,
iwconfig will show you the WEP key.)
If you are not connected to the WEP network, but want to find the WEP key anyway, there are tools to do this by eavesdropping on a bunch of traffic. I recommend aircrack-ng (see this tutorial on aircrack-ng). Here is a tutorial on using aircrack-ng with Backtrack to recover a WEP key. Or, you could use the link you provided.
WARNING WARNING WARNING: Do not do this without authorization of the owner of the wireless network. Hacking other people's wireless networks without their consent is highly illegal in most countries. Don't do it!
Old information from the previous iteration of the question (you can ignore this):
For WEP, if you only have the password and not the cryptographic key, then you will need to recompute the crypto key from the password. It is computed as a deterministic function, and if you Google for WEP key converter or WEP key calculator you should be able to find utilities that will do the conversion for you. Or, you can just go to your access point's web configuration, and it will usually tell the WEP key (indeed, you could enter in a new passphrase and see the resulting WEP key).
For WPA, you may need to capture the traffic associated with the initial association between the client and the access point (if you just come in later and capture it after the client has already connected to the access point, you may not be able to decrypt).