Over on Quaddicted.com, the instructions for custom maps in .PAK format is to use the "-game" command. But that command isn't even recognized by the game. another source says to use the syntax "game.exe -game" command but that doesn't work either.
Hey no worries. Quake custom content isn't quite as easy to use as in some modern games, but it's not super hard -- just requires a few pieces of info that you're lacking.
So I'll do a summary here, but I should also recommend this guide over at Steam: http://steamcommunity.com/sharedfiles/filedetails/?id=166554615
It's pretty comprehensive and also has links to some good launchers/frontends.
Anyway, if you download some custom content that is made up of multiple files, especially if they are pak files, it is probably meant to be used as a mod. (The readme that comes with the downloaded files will also probably talk about this.)
Quake has a particular way of dealing with mods -- for the most part, you have to indicate at the time you launch Quake
whether you want to "activate" a mod, and use that mod for your whole play session. Some modern Quake engines (source ports, like Quakespasm or DarkPlaces) also provide some support for switching mods while Quake is running, but that method can be different from engine to engine, and it also doesn't always work perfectly. So I'm just going to describe the "classic" way of activating a mod, which works with all Quake engines.
First, before you launch Quake, you need to have the mod installed properly. "Installing" a Quake mod just means putting its files into a folder, next to the id1 folder wherever you have Quake set up. Usually when you extract a mod from a zipfile it will create this folder for you, with all the necessary files inside, so you just have to make sure you put that whole folder into your Quake folder.
As an example: let's say you have a folder "C:\Games\Quake", and inside that folder is winquake.exe and/or glquake.exe or whatever Quake engine you are using. Also inside "C:\Games\Quake" in this example you would have a folder named "id1" that contains the original Quake campaign content. So your mod folder would also need to be placed inside "C:\Games\Quake", as a neighbor to all that other stuff.
Now when you launch Quake you need to tell it which mod folder to activate. This is done by passing command-line arguments to your Quake executable. The way to activate a mod is to use the command line arguments "-game mod_folder_name
". If your mod folder was named "travail" for example, you'd need to use the command-line argument "-game travail".
Command-line arguments are used for configuring various other things with Quake too, so it's handy to know how to do them. Usually, at least on Windows systems, you would make a shortcut to your Quake program and modify the shortcut to add the command-line arguments, or you would create a .bat file to launch your Quake program with the added command-line arguments. If you're not familiar with how to do this, there's a bit more detail in the section called "The Command Line" in this guide: http://steamcommunity.com/sharedfiles/filedetails/?id=120426294
Let us know if you can't get it to work for you.