So the time capsule is easy, connect a cable from one of the 4 switch ports on the back of your 655, to the WAN port on the time capsule. Then using either your imac or mbp, connected via one of the switch ports on the time capsule. Use the apple airport tool to configured you capsule, you want to setup a 5ghz 802.11n network, with WPA2-Personal, then on the internet tab where is says connection sharing, choose Off (bridge mode). So what this does is tells the time capsule to be an access point only and let the 655 do all the DHCP and internet connection for all clients.
As for the other, could be alot of things, try changing the channel to 1, 6 or 11, see if any help. It may just be that you can't get coverage corner to corner, so if you can move it to a more central location that would help.