While viewing UPlay dashboard [main-window] upon logging in, look towards top-left of program pane...
You should notice a button represented as a 'triple hyphen'; visually akin to the "EQUALS" = symbol with one more score.
Click that. Then hit 'Settings'.
The option to enable in-game overlay is near top of the first screen that pops up after entering the settings, under General.
. . .. . < SHIFT+F2 > is the UPlay overlay shortcut to access everything in-game.