We are working on a 64 Bit version, but do not yet have a release date.
At the moment you will need to install a 32 Bit Oracle11g Client on Windows 64 Bit. PL/SQL Developer is a 32 Bit application, so it cannot interface with a 64 Bit Oracle Client.
The 32 Bit Oracle Client can communicate with a 64 Bit Server, even on the same machine if necessary. The 32 Bit client can coexist with a 64 Bit Oracle installation when installed in separate Oracle Home directories.