when i connect the adsl cable coming from the wall in the wan port it never take an IP because it doesn't fit in the port.
Do not attempt to do this as it may damage the WAN port on the Time Capsule.
should i use a converter from RJ11 TO RJ45
The RJ11 phone line needs to connect to a DSL modem. Then an RJ45 Ethernet cable will connect from the modem to the Time Capsule.
The reason that you need a modem is that the Time Capsule is a router only...(.with a built in hard drive). It does not have a built in modem.
Contact your Internet Service Provider (ISP) , tell them that you have a router, and ask them to provide you with a modem to use with their service.