You can try slot 2 (as SW noted) and if that doesn't work you can try this in either slot:
1). turn on your PS1 with memory card in one of the slots.
2). Hold the memory card upwards. Push it upwards lightly with your finger.
3). You may have to manually tell the game to load the card from slot 1 or slot 2. Assuming the game recognizes your card is there, keep holding the card upwards until the load is finished
Sometimes, all these older cards need is a little push, to help them make better contact so the console can actually recognize and read them. I've got several cards that still work, but only if I do that little trick.
