Please refer to the UCL maps (or navigate there from the UCL page via "Information", "Where We Are"). On the "Gower Street Site" map you can locate the Council Room as follows:
At the bottom (South) of the main front quadrangle (square 3E) there is a block marked "South Wing"; the bump facing into the quad in the middle of this block is the window of the Council Room. Don't enter through the window, but via the east end of the South Wing block (where the small triangle is in the corner of the quad).
You can refer to the "General Area around UCL" map. Tube stations that are near UCL are Euston, Euston Square and Warren Street.
Go right (from the front of the main station) - i.e. west - along Euston Road until you get to Euston Square tube station. You can cross beneath Euston Road through the station. When you emerge at the other side... (go to continuation below)
Take the Gower Street exit... (go to continuation below)
Cross the main Tottenham Court Road (eastwards). Go leftwards (north) to the end of Tottenham Court Road and then turn right (east) along the south side of Euston Road until the right turn at the traffic lights. Cross the road (Gower Street) to Euston Square tube station... (go to continuation below)
...you will be at the top (north) end of Gower Street. Go down (southwards) Gower Street, crossing the first side road on your left (Gower Place). You will come to the main entrance to UCL on your left -- a gateway flanked by gatehouses into the main front quadrangle with a grand-looking building on the far side (raised terrace, steps and columns). Make for the far right corner of the quad (south-east) and enter the building in the corner. Inside the building go to your right, through the double doors that lead to the large staircase. Take the corridor to your right, and find the Council Room about half-way along the corridor on your right.