That was almost exclusively the situation with the first custom SP releases. And they didn't always even have a readme file. The .pk3 was just thrown in the base folder, and the map loaded via the console as has been suggested here. Later on mappers included custom menus so that you can start the game just like the original campaign: Clicking new game and choosing a difficulty.
The better levels are loaded as a mod. If you have patched your JO, the menus have a fully functional load mod option (unlike JA). If the level is intended to be loaded as a mod, you need to have it in a custom subfolder under Gamedata folder. You will find the folder name under load mod option in game. Decent SP mods always have custom menus to start a new game without any console commands.
To speak the truth, I don't think a level is worth playing unless it has a decent readme or (/and) custom menus to start the game easily.
Frozen in the past by ICARUS