eD2k web links are not added to eMule
If eD2k web links are not added to eMule when clicked on in the Internet Explorer, the special eD2k link format is not correctly associated. Check the eD2k Links button in Preferences -> General. If the button is inactive (greyed out) then some entries in Window's registry are messed up. Download this tool (by Resterampe) and remove the old association, then press the eD2k Link button again.

These registry entries are responsible for the web link format:
HKEY_CLASSES_ROOT\ed2k\shell\open\command
Name: (Standard)
Type: REG_SZ
Data: "Path to emule.exe(with quotes!)" %1

and

HKEY_LOCAL_MACHINE\SOFTWARE\Classes\ed2k\shell\open\command
same as above

Opera
Opera 7.2x and above
Run eMule. In Opera go to File -> Preferences -> Programs and paths -> Protocols and press the Add button. In the upcoming dialog enter ed2k in the Protocol field and the path to eMule automatically appears in the Open with default application. If it does not, the correct path to the eMule.exe will have to be provided in the Open with other application field.

Older Opera versions:
To allow weblinks to work from Opera, a configuration file in Opera's installation folder has to be edited. Quit Opera and open either one of the files in an editor, e.g. notepad.
Opera 6.x:
Add the line TrustedExternalURLProtocols=ed2k under [User Prefs] in the file Opera6.ini and safe it.
Opera 7.x:
For Opera 7.x add the same line in the file OperaDef6.ini.

Mozilla, Firefox
You will need a special plugin to associate different link formats like eD2k to the Mozialla or Firefox browser. Get mozex (alternate download link), open your browsers preferences and enter
path_to_eMule\emule.exe %r
in the eD2k edit box of the mozex plugin.

e.g.: C:\ProgramFiles\eMule\emule.exe %r