Yesterday I decided to get up to speed with alarms and have them forwarded to my e-mail account. All is working well (getting way more than I thought I would). For some reason now none of the triggered alarms are being displayed so I can't clear them. Not sure what I might have done to cause that.
Are you focused on the object on which the alarm was triggered. If it was at the virtual machine level, you need to view it on the VM...etc. Another place to look is under View->Management->Events
or from the top level of your vCenter instance, click on the Alarms tab. You will see any alarm that is currently triggered.