Pubnub example for Atmel SAM D21 and WINC1500 was a part of ASF, but is not in the latest version of ASF.
Look for code in earlier versions of ASF distribution, path
common/components/wifi/winc1500/pubnub_demo.
There is also an earlier "iteration" of this code in the history of this repository, which should work for some older ASF versions.
The Pubnub client is derived from the Contiki OS client, using the winc socket interface.
Its source code is in Pubnub.c. In the header (Pubnub.h) there are a few (documented) macros that the user can change to suit a particular application. For buffer/message size constraints, look at PUBNUB_BUF_MAXLEN and PUBNUB_REPLY_MAXLEN.
This Pubnub client support only the basic features: publish and subscribe to a channel (or a list of channels, but no channel group(s)) and leave (a (list of) channel(s)), which is suitable for most embedded applications.
Detailed instructions on how to connect all the HW, compile & upload the SW/FW and run the examples are in examples/README.md. Also, take a look at the application note in ASF.