Can't choose serial port (checked all the troubleshooting guide)

Arduino is registered on port ttyACM0 but i can't choose it from serial ports on the IDE because it's grayed out.
Can I tell the IDE to use ttyACM1 without the tools menu?
The board works( leds are on ).
I was able to upload sketches on the board.

The cable works and dmesg prints out:
[ 6527.535990] usb 1-1.2.1: Manufacturer: Arduino (www.arduino.cc)
[ 6527.535992] usb 1-1.2.1: SerialNumber: 9533130303435150C042
[ 6527.536285] cdc_acm 1-1.2.1:1.0: ttyACM0: USB ACM device

The IDE is launched with sudo arduino(Ubuntu 13).
sudo arduino
[sudo] password for user:
Experimental: JNI_OnLoad called.
Stable Library

Native lib Version = RXTX-2.1-7
Java lib Version = RXTX-2.1-7
Binary sketch size: 3,124 bytes (of a 32,256 byte maximum)
processing.app.SerialNotFoundException: Serial port '/dev/ttyACM1' not found. Did you select the right one from the Tools > Serial Port menu?
at processing.app.Serial.(Serial.java:191)

Found the solution!
In addition to all steps in the troubleshooting guide, it's worthy to check if it Java hasn't been altered.
I've found out that another user added a java package in JAVA_HOME/lib/ext to interface a board similar to Arduino, causing the IDE not to recognize Arduino when plugged in.
To proof check this kind of error you should try Arduino on a fresh operative system, or an operative system where java hasn't been altered.
Maybe this kind of error hopefully won't happen to anyone, but it's really hard to get since Java appeared to work fine, and the guilty java package didn't appear in the java exception stack trace.