What's the version of Dashboards Designer that you are using?
If you have bi 4.1 you can go with BIWS. For this, create a WEBI report and add a new variable. Specify the below formula:
=CurrentUser()
Add that parameter to the WEBI report and it will create a single cell block. Publish this block as BIWS and use this BIWS in your dashboard.
Regards,
Niraj