Here is a Google Street View link to where the lobby entrance of the building is: http://goo.gl/maps/poSns
Take either the Orange or Blue line to the Rosslyn stop.
To walk over from the station, here are simplified directions that should avoid the construction disruption and some odd intersections around the Metro stop, at the cost of a little bit more walking:
Once you get to the area, there is a lot of on-street parking, but I do not know how available it will be in the evening. There is 2, 3, and 12 hour parking in the area, all metered. There is no mobile payment for Arlington, everything is either meters (have quarters), or payment machines with a window receipt.
There is a parking garage under the building that is open until 11pm that you can use. I don't know if they have an after-5 special, but it's $7 for 1 hr, $12 for 2, and $15 for 3 during regular business hours. The entrance is on Nash St, right next to the loading dock and a Gold's Gym. Here's a Google Street View link that shows the entrance: http://goo.gl/maps/UbbkD.
After you take your ticket, you want to go down the ramp and then *TURN RIGHT* so that you park under 1400 Key Blvd. You should end up parked on either the D or E level. If you park on D, make sure you check out space 32D, that's where "Deep Throat" met Bob Woodward during Watergate(http://is.gd/S56HtN for more info). When you find a spot, take the elevator up to the 10th floor.
I believe the exit machines will accept credit cards, but there is also a payment machine in the elevator lobby of the C level in the other building.
If you have any more questions, or I missed something, shoot me an email or call