I know this is an old thread but it's one of the only places I can find anyone talking about getting openwrt/ddwrt running on a DIR-510L

I've spent the better part of the day researching and experimenting with mine and I've come up completely fruitless.

It's a very capable piece of hardware and was worth the $90 or so bucks I paid, but the more time I spend on the road the more I wish my OpenVPN connection was at the router level and not at the client level.

I'm going to give the HooToo TM04 a shot. If you check out the 3-25-2015 update in this thread it looks like these guys got it nailed. Talk about a swiss army knife.

https://forum.openwrt.org/viewtopic.php?id=53014