how do I pipe to the "for" command?

Although it doesn't appear to be documented, you can pipe to the command inside the FOR command. I first learned about it from this example:

SC query | FOR /F "tokens=2" %%i IN ('FIND /I "SERVICE_NAME"') DO @ECHO %%i

This modified version of your code works:

C:\>ipconfig | for /f "tokens=14" %f in ('findstr "Address"') do @echo %f
192.168.x.x
192.168.y.y

You can also use "more" inside the FOR command, as described in this answer, to parse input piped in from an arbitrary command:

C:\>ipconfig | findstr "Address" | for /f "tokens=14" %f in ('more') do @echo %f
192.168.x.x
192.168.y.y

Looking at the documentation for for (run for /? to see it), I don't think for /f will read from a pipe. The only valid sources are file-set, "string", and 'command'.

FOR /F ["options"] %variable IN (file-set) DO command [command-parameters]
FOR /F ["options"] %variable IN ("string") DO command [command-parameters]
FOR /F ["options"] %variable IN ('command') DO command [command-parameters]

You could put the command inside the for like this:

for /f "tokens=15" %f in ('ipconfig ^| findstr "Address"') do @echo %f

192.168.x.x
192.168.y.y

(thanks to neurolysis for pointing out it has to be ^| and not just |)

Or you could try putting ipconfig | findstr "Address" in a separate batch script and calling:

for /f "tokens=14" %f in ('ipaddresses') do @echo %f

(assuming you called the script ipaddresses).

Or you could use for to do the work of findstr too, like this:

for /f "tokens=2,14" %i in ('ipconfig') do @if %i==Address. @echo %j

192.168.x.x
192.168.y.y

it's not quite the same because it won't match IPv6 addresses, but something along those lines could work.

Also note that I changed tokens=15 to tokens=14. I think the last field on the line is token 14. Works for me anyway.