AdBlock Classic is an open-source extension for K-Meleon. It is an updated and de-branded descendant of the classic AdBlock Plus extension for Firefox. It included a custom interface for K-Meleon. AdBlock Classic supports AdBlock Plus filters and rules (including AdBlock Locus's Fallback subscription). The extension is open-source software available under the MPL 2.0. AdBlock Classic is distributed freely, as-is, and without warranty. That said, you're welcome to visit the K-Meleon forums if you have questions or issues.
Subscriptions are simply lists of blocking rules that are updated periodically. Most versions of K-Meleon will already come with some blocking rules for AdBlock Plus. Until you set up subscriptions, K-Meleon will use those default rules. It's best to choose a single base list (like EasyList) and then add as few supplemental lists as you can. You can configure subscriptions by going to Tools - AdBlock Classic - Settings to open the filter settings.
K-Meleon 74, the last version to run on Windows 2000, can run AdBlock Classic but cannot display the menu. The K-Meleon macro module (.kmm) below will add the menu. To install simply save it to your K-Meleon/macros/ or Profiles/your.profile/macros/ folder. Like all macros on K-Meleon you can open it in any text editor and modify it however you see fit. Current releases of K-Meleon do not require the macro. As the macro is based on Hermes' port of ABPrime, it can also be used to add a menu to ABPrime.