I always had the doubt whether you can attach JRMC with an running production application without re-starting the weblogic instances.
Here are the certain things you can do.
1) Check your license agreement with Oracle.
2) If not try downloading from http://licensecodes.oracle.com/bea.html. Look for the text "LIC-WLRT20" in this page.
3) Download the zip file named "LIC-WLRT20.zip".
4) Extract the file named LIC-WLRT20.txt and copy the same to %JROCKIT_HOME%/jre/
5) Rename the older license.bea file. In certain cases JRMC client will work with the existing version of the license.bea file itself. In that case you do not these above steps and you can proceed to download the JRMC client. If you swap the license file then you need to re-start the instances.
6) Download JRMC from the location http://www.oracle.com/technology/software/products/jrockit/index.html and install it (preferably jrmc-3.1.2-1.5.0) in your local machine say c:/jrmc
7) Ensure that the appropriate port number preferably 5 digit port numbers are allowed from your application servers.
8) run the following command %JROCKIT_HOME%/bin/jrcmd <process ID of the weblogic instance> start_management_server ssl=false authenticate=false port=<port number>.
9) Do a netstat -an | grep <port number>
10) You should see that the port number in LISTENING state.
11) Open C:\jrmc-3.1.2-1.5.0\bin\jrmc.exe
12) Create the connection to the application server. JRMC client will ask you for a password . Enter a master password atleast 8 characters long. Remember this password.
13) Test the connection and everything goes fine, you should be able to get to the console and start memory leak recordings.