There's some type of indoor racing thing at Vet's Auditorium going on tonight too, so parking will be tighter than normal but it still isn't that big a deal.
Normally onstreet and city ramps are free on the weekends however, you can't always be sure when there are events going on. I would guess there would be no charge tonight. Here's a link to downtown maps. Hit the link and then find the parking map:
Maps - Downtown Des Moines
If you want a ramp, the closest ramps that are on the skywalk, check out the ramps marked as #2 and #3.
We usually go out on Court Ave after events at Wells Fargo, so we always park down there and just walk. You can hit the skywalk at the Court Ave Brewery and take it all the way to the Well.