Great to hear you got it working.
WDS is strictly for bridging wireless clients. However, I think this build of DD-WRT, which is mini, because it has to fit onto 4 MB of ROM, doesn't have the WDS support. Go to Wireless > Basic Settings and under Wireless Mode see if you have anything with WDS.
Also, WDS is not a standard yet, and because of this chipset makers implemented it differently, leading to many incompatibilities between chipsets. DIR-615 E1 uses an Atheros chipset, while Tenda W307R (all revisions) uses Ralink chipsets. Problems are to be expected, even if the DIR-615 E1 DD-WRT build you have does support WDS.
If WDS is no go, then you are left with two options.
Option 1:
Use Tenda W307R in AP mode and the DIR-615 with DD-WRT in Client Bridge mode, with the blu-ray player connected over wired to it.
In this mode the blu-ray player will be on the same subnet, for example 192.168.1.X, with devices connected directly to the Tenda router. Not sure if that matters to you.
I'm fairly sure in this mode you won't be able to use the wireless on the DIR-615 to connect clients to it since it will be entirely dedicated for the connection to the Tenda W307R. I might be wrong though. You will, however, be able to connect wireless clients directly to Tenda and more wired clients to DIR-615.
Option 2:
Use DIR-615 as main router and AP and configure Tenda's wireless as "Station Mode." This will have pretty much the same effect on Tenda as the "Client Bridge" mode had on the DIR-615 in the first scenario, but the subnet will not be same.
Clients connected directly to DIR-615 will have 192.168.1.X IPs, while clients connected to Tenda, like the blu-ray, will have 192.168.2.X, for example. Accessing devices on 192.168.1.X subnet from the blu-ray (192.168.2.X) should be straight forward, but doing it the other way around would be problematic.
Also, in this option the blu-ray is also connected via a wired connection and the Tenda doesn't accept wireless connection from clients (I think).
So, without WDS support on the DIR-615 you can't really have the blu-ray connected via wireless, unless you link the two routers with a cable. But that deceives the purpose of your setup.
Also if different subnets are not an issue, I would choose option 2, because DIR-615 with DD-WRT would probably give your Internet connection more stability and will be able to handle more connections (like if you use torrents etc.) then Tenda.
Hope this helps. Looking forward on hearing what you chose and the progress made.