Chelsea Court is an area in Marleybone, which you will have access to after you finish Hyde Park, which is also in Marleybone. Just follow the storyline and you'll get there eventually.
You have to complete Hyde Park first. How could you not know that? Did you think you could just go anywhere you without completing anything?
If forgetno4 is a Theurgist or a Necromancer, it makes sense that he/she would want to ask how to get to Chelsea Court. Both Theurgists and Necromancers receive the same quest to defeat the same enemy in the same place. By the time they receive this quest, however, they will usually be finishing up Krokotopia and will as of yet not know a thing about Marleybone.
Now, take a wild guess which place in Marleybone this boss is.
