Whichever method you choose to create your hotspot, your wireless card will need to support either Ad-Hoc or AP mode. Examine the output of
iw list and look for the Supported interface modes: section. If Ad-Hoc or AP are not listed you may have to try installing another driver as there are several for Broadcom devices. You can view the name and version of the current driver in use with the
lshw -c network command.
Enable Hotspot (Ad-Hoc)
Select Network from System Settings. Select Wireless from the left hand panel. A Use as Hotspot... button will appear at the bottom of the dialog. Select it and you should now have a Wi-Fi hotspot utilising Ad-Hoc mode with your computers hostname as the SSID.
Note: By default this method will only use WEP encryption. You can modify this behavior by editing the network profile by hand.
sudo scratch-text-editor /etc/NetworkManager/system-connections/Hotspot in a terminal. Replace (or comment out each line with a #) the [802-11-wireless-security] section with the following.
psk=<your desired WPA keye>
Enable Hotspot (AP)
There is an alternative method detailed here allowing you to set up a Hotspot utilising the AP mode of your wireless device. One downside to using this method is that you may stop seeing other networks, presumably because it doesn't 'scan' once set in AP mode.