In the DefaultLoadOut.txt from the Scimitar download, the Scimitar line will include one column with the entry "NS." To activate the ship for both AIs and players, remove the NS, but DON'T DELETE THE TABS AROUND IT. Make sure you have a backup in case you delete too much. NS, or non-standard, is generally used for ships that appear in specific missions of the storyline campaigns. AI is used in that column to specify ships that aren't available to players, but are still given to D3 AIs.
If you ONLY change the DefaultLoadOut.txt (and not the DafaultCore.txt), players should be able to get the Scimitar without any trouble. However, they will have to make sure they get the Scimitar model folder to be able to see it in-game.