Hello,
I have an existing Linksys network set up for two PC's. How do I add laptop to the existing network?
Thanks in advance for your help here.

Your setup implies you have a Linksys Router? Which serves internet to both PCs, or just a hub/switch? To share resources between PCs?

How do want to connect the laptop, wirelessly? Or wired? If wired, simply plug an ethernet cable between the laptop ethernet jack and an available ethernet jack on the hub or router.

If you want to connect wirelessly, you need a wireless access point OR a wireless router. What Linksys model do you have?

If you have a Linksys wired router, it should have 4 LAN ports. Just connect the laptop to one of those ports and run the Networking wizard.

If it's a wireless router and you want to connect wireless, still run the Networking wizard and follow instructions.