Thank you for the help! The code you gave didn't work so I followed your instructions and looked up the code listed for the Enter key which is what it is labeled on the TV remote. I tried the first one and, son-of-a-gun, it works!

For anyone else who needs it, this is how I got my comcast gray remote with the red OK button to be able to enter the input source on the Sceptre TV set:

Press TV
Press Setup until 2 blinks
Enter 994 (this is the code for the OK button on the Comcast remote)
Press Setup
Enter 00140 (this is the code that the Sceptre TV remote sends for the Enter button)
Press Select/OK.

To use the remote to select the input source, you need to push the TV button at the top to enter TV mode, then push the button under the number 7 labelled "Input" and "TV/VCR". Now you can pick the input source using the up and down arrow keys and, finally, select that input using the red OK button which now sends the code expected by the Sceptre TV.

No one at either Comcast or Sceptre was able to provide this critical piece of information - they each kept pointing at the other.