The original "Poke646" was designed long before Steam became the platform for Valve's games and has some bugs when being run under Steam. In order to install and get the full potential out of Poke646 under Steam, please follow these few steps:

Make sure you have the latest version of Steam running on your system.

Download "Poke646" v1.0 and run the installer. When being asked where to install the game, place it in your "Steam/SteamApps/<your_mail_address>/Half-Life" directory.

Download the "Poke646" Steamfix and unzip its content into your "Steam/SteamApps/<your_mail_address>/Half-Life" directory. When being asked to overwrite the existing "kb_act.lst", do so.

Restart Steam and you should see "Poke646" listed under the "My Games" tab, from where you can create shortcuts for the game or directly launch it.