As Kerosene says above good tips for people struggling with weight editing.
The way trainers work (so the way users should think) is unless something is changed in the game coding then you cant change it in a trainer.
So if you wish something to change for example weight then you need that number to be changable (not hard coded like it starts of to be in the game {unchangable})
The way this changes is the explaination above by Kerosene get a INQ perk that removes the hard coded value which was set by the game now making it editable.
If you understand this principle you will understand why trainers do not work for you but do for others. Its that they understand certain values need to be changed to allow the trainer to work.
Some values are not hard coded and thus only need incrimental change to allow them to be edited for example money.
If you have some money then change the amount by buying something you have changed the starting value and thus allow the trainer which is a very well written GUI to edit the new value.
In short if others are getting it to work its something you are not doing correctly
or something you need to find out by reading the thread.
If you have the P.C version change the mapping of F1, F2 etc to something else then you can use the trainer in game at any time the advice by M.r Antifun is because if you wish to use the trainer it will conflict with default bound buttons in game. That is why he suggests using the trainer at load screen. Also there are some games that load screen is the only way for them to work as they have either cheat protection or the memory addresses need to be non resident in game.
One important thing if You stop the trainer (I dont mean F1 I mean click the X top right ) you will need to come out of the game completely not just to menu as it will need you to restart the game fully then initiate the trainer again.
As always ignore my English mistakes I do