If you've not already got the option to turn the DRL on on your instrument panel (instructions should be in the user manual on how to do this), then this option will need to initially be turned on by a diagnostics computer (Diagbox for modern Cits, Lexia on older ones). If your longer term plan is to do some of your own maintenance work on your C5, or you just don't trust a main stealer when they tell you what's broken and what needs fixing, then you should consider getting your own Diagbox/Lexia kit which will enable you to turn on the DRL, check for faulty components, reset various other options, and quite frankly do a whole host of other extremely useful things. Various clone Diagbox kits are available, but a lot of people have very good things to say about Jim at Easydiagnostics
http://www.easydiagnostics.co.uk/" onclick="window.open(this.href);return false; complete with his new whizzo website
That's where I got my Diagbox from, and it all works well. You'll also need your own laptop to run the Diagbox on, but this doesn't need to be anything special - i'm running mine on a donkey old Dell D400. Jim's Diagboxs run on Windows XP/7. The other - admittedly easier - option, is to get your mechanic to turn on the DRL, but they'd need to be a Citroen specialist with a Diagbox diagnostics computer to be able to do this.
Hope that helps.