Magnus isn't a Prime dude. So he can't use the forge. But Optimus can.

 

your confused. Megatronus prime was one of the thirteen. The megatron we no named himself after him and shortened his name. megatronus prime is a prime megatron isn't. only one chosen as a prime can use the forge. megatron tried but it didn't work.

 

So then why did Orion Pax called Megatron Megatronus at the end of Season 1?
He simply said "Where are we Megatronus?"

 

"As Megatronus left the gladiatorial arena for the political, he saw fit to shorten his name." - Ratchet, One Shall Rise part 3

Megatron originally took "Megatronus" as his name, eventually dropping the "-us" from the end. Orion's memory was reversed to before this event happened, thus he still knew his old friend as Megatronus.

 

^ read this

 

Megatron stole the other guy's name.

 

my dirty mind will get me in so much trouble one day.....

 

What? No.

Megatronus Prime is the 'Fallen', he IS a Prime. The Decepticon leader was originally D-16, changed his gladiator name to "Megatronus", then shortened to "Megatron" as we know him now; he is NOT a Prime.