From your email I'm guessing you were confused by what the hifi shop told you. There are two ways you can connect your sub, RCA line level connections and speaker connections. It sounds like you are trying to do both.

If you want to use the RCA connection, run an RCA cable between the sub-out on the receiver to the RCA input of the powered sub.

If this isn't convenient and you want to use the speaker level inputs, run the speaker wires to the sub and connect to the R/L input satellite speaker connections on the sub, then run speaker wires from the sub's speaker level outputs back to the satellite speakers. The sub will take the input from the receiver, filter out the mid/highs and amplify the sub frequencies.