How do you install the Arduino IDE on the Raspberry Pi running an ARM version of Ubuntu or Raspbian? There doesn't seem to be any official ARM release. Both distros have an "arduino" package in their standard repos, but it only includes an ancient 1.0 version that's missing a lot of features.
The source code is available on the website you listed. You should be able to install the pre-requisite software, (gcc-avr, avrdude, avr-libc, and avr-evtd) and then compile the source code.
While entirely possible, this will probably take a long time. If you're using an RPi 2, make sure to take advantage of the multiple cores by using
make -j8 instead of
make. Even so, compilation will take a long time; be prepared to wait.