Okay here we go.
When you open the Mass Edit screen you'll see the screen where you can enter the values that will be set to the targeted players. I reckon you want to edit certain players so you click the "Press me To Filter" in the bottom right.
The screen that appears works exactly the same as a regular search screen in FMRTE. So if you want to change players of a certain team you go to "Contract" and select the team at "Team". If you're finished building your selection you press "Search" on the bottom left.
You'll return to the Mass Edit screen where the "Press me To Filter" has changed to the number of player resulting from your selection. If you want to see what players where selected you'll have to do a regular search first.
Now you can set the values of the attributes you want to have for your player selection. You'll find stamina and strength on the "Physical/Hidden" tab.
Press "Apply" to mass edit your players.
Good luck and have fun.