Since the man will move together with the box, he will climb the ramp as well, therefore he needs to add an extra force that is equal to the component of his own weight in the ramp direction.
The image below illustrates the forces:
So, the force F is applied on the box, but also an additional force equal to Wx needs to be applied in the man, so he can also climb the ramp:
[tex]\begin{gathered} W=m\cdot g \\ Wx=W\cdot\sin (\theta) \\ Wx=m\cdot g\cdot\sin (\theta) \end{gathered}[/tex]
Why use sine and not cosine in the formula? The diagram below explains when to use each one:
