Where to find what’s appened when JDeveloper is creating the « IntegratedWebLogic Server » domain ?

The answer is quite simple:
$FMW_Home/Oracle_Home/logs/wlst_YYYYMMDDHHMMSS.log
Where FMW_HOME is the location of the SOA Suite 12c Installation
for example:
/u01/app/oracle/Middleware/Oracle_Home/logs/wlst_20141217130953.log

Where to find the IntegratedWebLogic Server log ?

As usual logs are located in the servers directory:
$JDEV_USER_DIR/system12.1.3.0.41.140521.1008/DefaultDomain/servers/DefaultServer/logs/DefaultServer.log
Where $JDEV_USER_DIR is the location of the direcory where jdev will create user files (projects…)
for example:
/home/oracle/domains/DefaultDomain/system12.1.3.0.41.140521.1008/DefaultDomain/servers/DefaultServer/logs