How do I get a memory card n64 to work if the game thinks it's not there?

If you mean for save games, sort of, the games saves are stored on the cartridge but the memory card allows you to transfer data between cartridges and also save ghost data (apparently).

But if want to, you could pick up the N64 Expansion pack for it, adding 4mb extra RAM. (actually a lot to the 64.) You have to get the expansion pack if you want to play games like Majora's Mask. Ocarina of Time is better though.