I would do something along the lines of....
OnActivate>Self>VCEmodVariable> [random] [==]
OnActivate>Self>VCEmodVariable> [random] [randomNumber] [5]
OnActivate>Self>VCEifVariable> [random] [==]
OnVariableTrue>Player>Additem>Whatever
OnActivate>Self>VCEifVariable> [random] [==] [1] [5 5]
OnVariableTrue>Player>Additem>Whatever
OnActivate>Self>VCEifVariable> [random] [==] [2] [7 7]
OnVariableTrue>Player>Additem>Whatever
OnActivate>Self>VCEifVariable> [random] [==] [3] [9 9]
OnVariableTrue>Player>Additem>Whatever
OnActivate>Self>VCEifVariable> [random] [==] [4][11 11]
OnVariableTrue>Player>Additem>Whatever
OnActivate>Self>VCEifVariable> [random] [==] [5] [13 13]
OnVariableTrue>Player>Additem>Whatever
First 2 events make it a random number between 0 and 5
Rest are basically, depending on the number generated, give certain item. There is probably easier way to do this, but this is the way I would do it. The [9 9] etc are the subsystem... If you don't know what that is, I suggest looking at the VCE guide.