Player Creation

The mode you are looking for is the Connected Franchise mode. You can start it as a current player, legend, or created player, as well as being a owner or coach. Here is a link with a little info on the mode. 

http://m.ign.com/wikis/madden-15/Be_a_Player

Oh, my bad. I will check and see if I can figure it out when I get home in a few hours.  I will let you know if I figure it out. 

OK I just created a 99 OVR Defensive end for the 49ers named Ivan Drago to see if I could make this work. Seems to work just fine.  After creating him I saved the roster as "trial" then without loading any other roster I went straight to "Play Now".  Just before kickoff I paused it to see if he was on my team, and even though he wasn't in my starting lineup, he was on my roster, and I was able to sub him in without an issue.  If you don't load another roster, and are only playing offline, this should work for you.

You can then go into "Manage Rosters" under the customize tab and move your new player into the starting lineup on whatever team you assigned him to.  Seemed to work flawless for me.

I doubt it because it sounds like you and I downloaded the same roster file. The following is step by step how I put a created player on a custom roster. 

You have to go into the "Manage Files" option in the "Customize" tab.

Select "Load and Delete Files",

Tab over to "Roster" and select the one you downloaded.

Go create your player

When finished creating the player, select the roster that you want him to be on (the one you downloaded)

Select "Yes" to overwrite the roster

The new roster should be loaded already with your player on the team you put him on, but you'll need to go into the Manage Rosters tab in order to make him a starter.

Select "order depth chart" and place him in the starting position you want him in. Back out and select "Manage Files". Select "Save", Tab over to Roster and save it either as a new roster, or over the top of the roster that you downloaded before.

Tab over to the "Play" tab and select "Play Now"

Pick the team that you put the created player on as the team you will be controlling.

Start the game and your created player should be in the position you wanted him on the team you wanted with the custom roster that you downloaded. I just did all of these steps in order and typed this post as I did each step so there shouldn't be anything missing.

Nope, on Xbox 1, sequence shouldn't be any different though. I did it twice last night with different players on different teams and different rosters. I think the key is downloading or loading the roster you want to use before you create the player. Then pay close attention to the team and position your created player is created in. 

It should work if you follow the sequence I gave you as long as you are playing offline games (just play now against the computer or an opponent using another controller on your system.). Online games are only able to be played with the most current roster available.