So, as known from the error message, class ServImp_Stub is not found!
Now, the next question would be "what the #$%^ is this class all about"
Stub classes are used by RMI to communicate between two JVMs.
JDK 1.5 onwards you do not need to generate these classes instead RMI uses dynamic proxies to generate stubs at runtime for you. There are some ifs and buts that you can find out either by searching this forum or google.
If you are before JDK 1.5, then you need to generate the stub classes using rmic.
I hope you have gone through
Sun RMI Tutorial, if not, kindly do that.