Can't Change ModSim's Clock Period

In ModSim's Wave window, when I right click on a signal name and select "Clock", the "Define Clock" pop up appears. In this pop up I enter a value into the "Period" field and click on "OK". The pop up dissappears without error and so it appears the value has been accepted. But when I reopen the "Define Clock" pop up the value I put in the "Period" field is gone and the former value of 100 is there. I cannot seem to change this value to other than 100. How is this done?

I am using ModSim 12.0 with Quartus 12.0sp2 Web.