I just recently had the exact same issue. All of the buttons would learn with the exception of the "Enter" button.
There is a simple solution, however. In the NeoEdit software make sure that the device is VCR and the second drop down selector should be Tivo. Then select one of the two possible codes that are listed for the Tivo.
Once you have done this, open the remote control window that has the "enter" key that you were previously trying to "train" and edit the button by double-clicking on it.
This will bring up the dialog box which contains all the properties for this button. In the first tab called "Actions", make sure that you are sending an IR-Code and then double-click on the work IR-Code. Use the drop-down selector on the box that pops up and select "Enter".
Viola! That's all there is to it.
I created an ncf file that contains my TIVO selections at:
[Link: evisions.com]Take a look at it and post another reply if you have any other questions.