I just did a little research on your laptop. Unfortunately it looks like the RAM on the whole X540 lineup is soldered to the motherboard and cannot be replaced or upgraded. The reason task manager shows you have one slot is because Windows has no way of knowing if RAM is soldered or socketed. If you need more RAM, you will need to get a different laptop, even if you got another motherboard they only ever made them with 4GB of RAM from my research.