You need to use crtl+alt+pause since crtl+alt+del is handled as a special combination on the host windows machine. You also need to set Synergy to auto start on the client PC. From the Synergy FAQ:
Synergy isn't able to capture ctrl+alt+del on PC compatible primary screens because it's handled completely differently than other keystrokes. However, when the mouse is on a client screen, pressing ctrl+alt+pause will simulate ctrl+alt+del on the client. (A client running on Windows NT, 2000, or XP must be configured to autostart when the computer starts for this to work.)
On a primary screen running on an OS X system, you can use ctrl+command+del. Using the pause key isn't necessary since OS X doesn't treat ctrl+command+del differently. And using the pause key isn't usually possible because there isn't one on most OS X systems. Use command instead of option/alt because the command key, not the option/alt key, maps to alt on windows. The reason is because the command key is in the same physical location and performs the same general function (menu shortcuts) as alt on a windows system. This mapping can be modified in the configuration.