After you beat the king triceratops, if you fly to the southern part of west city, you'll meet cooler (frieza's brother) he'll say that he would wait for goku in new namek.
But where is new namek well, you need to find all the seven missing nameks (you'll get this mission from the scientist at the capsule corporation). If you found them all, go to the spacecraft at the capsule corporation, go inside, the screen turns black,then go outside,you'll be at new namek. go north and you'll find cooler inside...