I don't understand how to start a new game.
It is impossible to power the Synthesizer to produce matter.
Is this a mistake or am I missing the genius of the mod?
Give a hint to start the game, if that's how it's intended.

You can craft basic matter while holding the cube in your hands. With this you can make a boiler and put the cube in it to get steam.

You can craft basic matter while holding the cube in your hands. With this you can make a boiler and put the cube in it to get steam.

https://prnt.sc/6kFyO624guj3

Do you have any other mods that could be interfering? This shows "synthesizer + character" for me

Yes, it was https://mods.factorio.com/mod/GirlCharacter
Not at all obvious at first glance.

OK, I am pretty sure it is because Ultracube adds some non-default recipe categories to the crafting categories of the main character prototype. This isn't really avoidable, I'll see if there is a way to make it compatible.

In 0.2.4 you should be able to use the Gear Girl Character mod without it causing problems.