I thought that this user would be set in /etc/i-doit/i-doit.sh so I did set it.
/usr/local/bin is the path suggested by the Knowledge base article linked above, as are the file names. If your i-doit sysadmin decided to place the files somewhere else or name them differently, you have to find them in your local system and act accordingly.
One file (named idoit-jobs in the Knowledge base article) executes various tasks on the i-doit database, and the other one (named idoit) implements the basic i-doit console invocation. The former calls the latter (which you can see by the enviroment variable CONSOLE_BIN set in the file).