Assuming, by "black cable" you're referring to the coaxial output from the cable set top box.
If this is the black cable you are referring to, make sure the cable from the wall outlet is connected to the set top box input and the second cable is connected to your set top box output and your Element TV. Once the two are connected, turn on your set top box to any channel (doesn't really matter as you can't see it anyway). Just pick a channel you know you should be receiving.
Go into your TV's set up and perform a channel scan. This should pick up one channel, your set top box output. With your TV set on that channel, you'll be able to view any channel your set top box outputs. In other words, change channels using the set top box and not the TV.
The composite (Red/White/Yellow) connections should work much easier. You just need to make sure the cables are connected properly and as they're color coded, that shouldn't be a problem. Just make sure that if you are using this type of connection, that your TV is tuned to the Composite input.