Unfortunately that was an old installation guide (I haven't updated all posts on the forum, just too much work :-(. Cosa is now packaged as an Arduino core and much easier to install. The procedure is in the first post on this topic and in the blog. And below (why wait ;-).
1. Download the Cosa zip from github.
2. Unzip the file in your sketchbook/hardware directory. If you do not have a folder named "hardware" in your sketchbook directory please create one before unzipping. If you want rename "Cosa-master" the default git download to "Cosa". In Windows simply drag the Cosa-master directory from the zip file into the sketchbook/hardware directory.
3. Restart/start Arduino IDE and Cosa will show up as a set of examples and boards.
1. Connect an Arduino board.
2. Select the Cosa board setting and serial port in the tools menu.
3. Compile the empty sketch.
4. And see the default setup and loop in action - an event driven blink sketch. Here is the code for the empty sketch (all defaults). https://github.com/mikaelpatel/Cosa/blob/master/cores/cosa/main.cpp
I will have to get back to you with a guide on how to mix Arduino code with Cosa. This was possible "out of the box" before the packaging as a core. Now there is a bit more moving into directories and there might be some extra stuff. I will be back with the details.
BW: The compiler errors that you are getting are from the mixing of Cosa with Arduino/Wiring in the same sketch. You will need to compile Arduino or Cosa in separate files (classes). The conflicting symbols in your sketch are LED and A0. These are defined in Cosa board (as enum values) and they are define/variable in Arduino pins_arduino.h. Cosa is not really yet another Arduino library. It is more an OOP framework for small scale embedded systems. Cosa offers a clear structure, "architecture", for adding device drivers, etc. It does not use malloc/new, free/delete and is very conservative on SRAM usage. Also the whole framework, all classes, compiles from Tiny to Mighty and Mega. That includes all TWI and SPI drivers, LCD, etc.
What other libraries are you using?
PS: The alternative installation, if you interested in following the project more in detail and know a bit about how git works, is:
1. Create the hardware folder in your sketchbook
2. git clone from github in that directory
3. Frequently do a git pull to keep up to date with the development. Cosa is still moving with new libraries, drivers, etc. More or less weekly additions. Todays update is a Touch sensor. Don't forget to "Watch/Star" on github.