Where is Commander Bailey's assistant?

Solution 1:

While Bailey himself is on the Citadel embassy floor, Bailey's assistant is on a different floor of the Citadel, the Presidium Commons. He's at the C-Sec outpost there, marked with (5) on the following map:

The assistant is the Turian sitting at the desk:

Solution 2:

When you enter the Presidium, turn right, go forward until you find the first wall (a little past the Vorcha talking to the woman about a billboard they're watching), and continue bordering the wall it.
You'll go a little "outside", down some steps, and the C-Sec office will be on your right.

Commander Bailey's assistant will be sitting at his desk.