I've just looked at a map and I'd reckon on about 45 minutes fairly brisk walk.
Easier: Cross the Seine using the bridge which is next to the Eiffel Tower. (This is the Pont d'Iena). On arrival on the north bank, turn right along the Avenue de New-York. After a minute or two of walking you'll see Avenue Albert de Mun on your left. Turn up here and then take the 2nd right (Avenue d'Iena). Iena Metro station is a short way along, on the right.
Take Metro Line 9 southbound. (Train destination = Pont de S�vres). Alight after 9 stops at Porte de St-Cloud. The Parc des Princes is a few minutes walk away. It should be sign-posted but for a major event you can just follow the crowds!
Alternatively, cross the Seine, as above, and take Bus No. 72 (Destination board shows 'Parc de St-Cloud'). This runs along the Avenue de New-York, keeping the Seine on its left, and takes you very close to the Parc des Princes.
If you're travelling round Paris for the day, make sure you buy a 2-zone Mobilis ticket for unlimited travel. (If you're going by Eurostar, don't be tempted by a similar ticket which they sell at Waterloo. This is a 3-zone ticket, which you won't need, and costs much more).
Chris