Access workshop from within game, its the only way it works at this time. After you sub start a new game, open ship designer in game, click on New Design then click on Downloaded tab.

it didnt work

Click on New Game to open Civilization Builder. Click on any civilization you're going to add ships to. At bottom click on Workshop button. After you sub start a new game, open ship designer in game, click on New Design then click on Downloaded tab. Subbed designs appear there, To use in game equipe with weapons and defenses and save as Ship Design

Also check the type. On a new game, you probably won't see any custom designs that are Small or bigger type.