Web functionality is included in the regular AirDC++ desktop application.
After you have downloaded the client, you may enable web functionality from
Portable binaries for Linux
Precompiled versions are available for Linux.
Note that the binaries are not compatible with FreeBSD or other non-Linux platforms.
There is no installation package available at the moment but developers are able to compile the client from source. See this issue for more information about the OS X package status.