Call RAND(1,B). "B" is the count of the list. This number will be used to get an item from the list, using the random number as the index.
Use double %% for your list variable (something like %%Variable%%VarName[index to be calculated by RAND() function]%%) and add a "Process Tokens" action, because the index itself is a variable to be processed first.
KM will first process the RAND() function and make double %% into one %. The process tokens action then processes the variable.