What do i need to press if i want to draw the inverse of f(x)=x+5x^4 for example?
The only thing I can think of at the moment is this: write the equation as parametric equations. Since $\displaystyle y=x+5x^4$, the inverse would be $\displaystyle x=y+5y^4$. Now, if we left $\displaystyle y=t$, then $\displaystyle x=t+5t^4$.
You can now plug this into your calculator. Make sure you're in parametric mode!