problems with Arduino IDE in Ubuntu

hi
when I install the Arduino IDE on ubuntu it doen't work propely. There aren't any serial port aviable the sereial port item of the tools menu isn't active. It doen't work with any board, I tiried to install it on an other computer with ubuntu and it doesn't work at all. When I lauch Arduino for the first time it ask me to add external plugin but it doesn't work neither with the plugin installed or with the plugin not installed.
thanks for the advice and sorry for my english