"WC" stands for "water closet," which is another name for a bathroom.

In Anne Frank: Diary of a Young Girl, the living quarters in the Secret Annex are cramped. They live above a business; access to the second floor and attic lies behind a hidden door. During work hours, the hidden inhabitants cannot make noise, wear shoes or run the water.

In terms of the WC, there is a toilet in the "flat," but since no water can be used during the daytime, it cannot be flushed until the office closes for the day. When Mrs. van Daan arrives, she is carrying a chamber pot with her:

Much to our amusement, Mrs. van Daan was carrying a hatbox with a large chamber pot inside. "I just don't feel at home without my chamber pot," she exclaimed, and it was the first item to find a permanent place under the divan.

Once, when the plumber had to service the pipes downstairs, other "chamber pots" were created: 

...I improvised a chamber pot, sacrificing a canning jar for this purpose. For the duration of the plumber's visit, canning jars were put into service during the daytime to hold our calls of nature.

When the office beneath them is burglarized, the occupants of the annex cannot move until the danger is over so...

Next we did three things: we went over again what we supposed had happened, we trembled with fear, and we had to go to the lavatory. The buckets were in the attic, so all we had was Peter’s tin wastepaper basket.

Though it is an unusual topic, it is a real part of any living creature's existence; special provisions had to be made for all of the families and inhabitants living in the annex—when no other options were available to them.

Additional Source:

http://www.trhs.weber.k12.ut.us/hatch/my%20local%20web%20files%203/Portfolio/documents/powerpoint/diary_of_a_young_girl.pdf