If you go into Preferences->Key Bindings - Default, you can find all the shortcuts, below are the lines for commenting.
Code:
{ "keys": ["ctrl+/"], "command": "toggle_comment", "args": { "block": false } },
{ "keys": ["ctrl+shift+/"], "command": "toggle_comment", "args": { "block": true } },
edit: change the keys to what your preferences are, or try:
ctrl+shift+# to toggle block a comment or ctrl+# to toggle a line comment