JEJoll :
I Have an old DLink DIR-615. I would like to connect it wirelessly to my current network in order to be able to use it as an access point via an ethernet cable for another machine in another room that isn't wifi enabled.
The other replies are confusing because you're mixing up terminology.
■A wireless access point is a device which you stick on to your LAN, which creates a WiFi network connected to your LAN. The instructions in your link are for turning a WiFi router into a WiFi access point.
■A wireless adapter is a device which you connect (via ethernet, USB, or as a PCIe card) to a computer without WiFi, which then allows the computer to connect to WiFi networks.
From the end of the quote above, it sounds like you want to use the DIR-615 as a wireless adapter, not an access point. It is possible to turn it into a wireless adapter if the device supports something called bridged or client bridge mode. When you put it into this mode, instead of creating a WiFi network, it uses the WiFi hardware to connect to an existing network.
■In bridged mode, the WiFi network it connects to will show up as if it were plugged into a LAN port, and devices you plug into the DIR-615's LAN ports will be able to connect to your primary LAN (by hopping over WiFi). The DIR-615 is "bridging" its LAN ports and the primary LAN together over WiFi.
■Client bridged mode is like plugging the WiFi network into the router's WAN port. Devices you plug into the DIR-615's LAN ports will be able to access devices on your primary LAN (and the Internet if your LAN has Internet access), but the DIR-615's firewall will prevent devices on your primary LAN from accessing devices plugged into the DIR-615's LAN. So this is suitable if you just want Internet access. But if you plan to share files within your LAN, you want regular bridged mode.
If the DIR-615 does not natively support bridged mode, you can usually add the capability with third party firmware like DD-WRT. From a cursory glance, it does look like DD-WRT can be installed on the DIR-615, but there have been multiple versions of the DIR-615 released. You're going to have to do some legwork to figure out if your exact version is supported, and how to install DD-WRT onto it (which is not always an easy task). Furthermore, be aware that DD-WRT has had a long-standing bug where bridge mode does not start working until after you reboot the router. That is, you set up bridged mode, configure it and connect it to a WiFi network, and it will not work. Reboot the router, and suddenly it will start working.
Usually the reason people do this is so they can connect *multiple* ethernet devices into the router-as-a-wireless-adapter. You can do it for one device, but if the DIR-615 doesn't support bridged mode natively, the amount of work necessary to install DD-WRT and set it up in bridged mode may make you wish you'd just bought a USB or PCIe wireless adapter instead. OTOH, the antenna on the router tends to be better at pulling in a wireless signal than a dongle and even many PCIe wireless adapter cards. So this might be a preferable solution if your computer is in a part of the house with a weak WiFi signal (bearing in mind the DIR-615 may not support the latest wireless protocols and speeds).