That don't solved my problem.
When do you get this message? Please share more detail and also the screen shot of the Data Visualization Module form, search Module Name = "AtriumWidget". turn on the java plugin logs in debug mode and look in arjavaplugin.log when this error occurs.
Please look at the similar thread here ARERR  The requested plug-in "AtriumWidget" does not exist in the list of Data Visualization Module Server(s) in the BMC Remedy Mid Tier Configuration Tool
are you still facing this issue?
I tryed that, but isn't solved my issue.
But I contact BMC and after 3 days of supporting, we have find a solution.
The parameter "RPC-Client-XDR-Size-Limit: 450000" was missing in ARS config file(ar.conf) and this was causing underflow when midtier tryed to download plugins.jars files at ARS.
I put the parameter in ar.conf file , restart midtier and this solved my issue!!
Thanks all for support.