Here's what gandreas taught me.
First of all, numeric or textual?
To create a custom die, tap on the Plus under the type of die you want.
If numeric, you get a Number6 die. Change the number of sides, the name, and description. The name should end with the number of sides. Go down and edit the Label and value of each of the sides. Same goes for a textual die.
My suggestion would be to name it with just a few letters and nothing that might be confused with a command. My D&D Avenger character rolls 3 instead of 1 and 2, so I created A8. Then I replaced 1 and 2 with the value 3. I also have a custom die for the players (so I can determine who to attack) is named Party6.
In the custom editor, the dice will appear with d or t in front of the name you give it. So the dice will appear as tParty6 or dA8.
To roll these dice in the formula, enter "dA8" in a regular keyboard. If you are using the custom keyboard, tap Dice then d?. Then tap double tap on the entry. A window will appear asking if you want a custom numeric die or enter the number of sides. If you tap on Custom numeric die, a scroll window will appear with all of the custom dice you've created, plus a few that gandreas has added.
If you created a textual dice, then tap Dice then alt. This will change the d? to t?
To roll them from the dice bar, do the same steps as above for adding it to a formula.
Clear as mud?