'mongod wiredTiger' on Ubuntu?
I wasn't able to use wiredTiger upgrading MongoDB. However, at May 17 I uninstalled MongoDB 2.6 then installed MongoDB 3.0.3 . Immediate after the installation, I added storageEngine=wiredTiger on top of my mongod.conf file. Then I gave sudo service mongod start command and eventually I could.



Edit:

For fresh installed as directed by official documentation;


Open configuration file using sudo nano /etc/mongod.conf
Change the #  engine line to engine: wiredTiger like the below
Run mongod using the command sudo service mongod start


.

# Where and how to store data.
storage:
  dbPath: /var/lib/mongodb
  journal:
    enabled: true
  engine: wiredTiger
#  mmapv1:
#  wiredTiger:




Edit:

If the current version is supported the wiredTiger;


Get the backup of the current database using mongodump command
Stop the mongod service using sudo service mongod stop command
Add storageEngine=wiredTiger text as the first line of mongod.conf file
Delete the all file on /var/lib/mongodb (or /data/db folder if used)
[This is important. Because MongoDB cannot convert the current MMAP db files to wiredTiger format]
Start the mongod service using sudo service mongod start command 
Restore the database from the backup using [mongorestore][2] command
wiredTiger is being used...