Aleks does not offically support ubuntu,but you can follow below steps to install aleks plugin in ubuntu manually,you need to install Oracle Java JRE(Not Open JDK) to support this plugin.
2.Download alekspack10.JAR from http://www.aleks/downloads/linux_jvm
3.Open a Terminal by pressing Ctl+Alt+t or search terminal in Dash
4.Move or copy aleksPack10.jar to your /usr/lib/jvm/jre_<Ver>/jre/lib/ext using below command
sudo mv aleksPack10.jar /usr/lib/jvm/jre_1.7_04/jre/lib/ext
5.Done.You just installed aleks plugin in ubuntu,restart your firefox and try to access Aleks website again.