For MyEclipse 10.x
1) Restart MyEclipse if it is already running.
2) Download the attached patch archive (you need to be logged in to see the file).
3) From the MyEclipse Configuration Center, go to the Software tab and choose "add site".
4) Click "Add from Archive File" and select the zip you downloaded. Use any name you find appropriate. Note, you don't need to add anything from this site, it only needs to be present at the time of resolution.
5) Go ahead and attempt to install the Activiti Designer from their update site, it should install without any further issues.

Please note that this currently leads to the installation of graphiti 0.9.2 into MyEclipse 10.x, while MyEclipse 10.x ships with Graphiti 0.8.0.
Version 0.8.0 is incompatible with 0.9.x, as a result, the JPA Diagram Editor in MyEclipse will stop functioning; this is why the patch is not being made automatically available. Unfortunately there is nothing we can do about this incompatibility, the 2013 SR1 release will not have this problem.

For MyEclipse 2013
There is no workaround for installing the designer into MyEclipse 2013 GA (2013 GA is based on version 0.9.0 of Graphiti while the designer currently requires 0.9.2). However, the upcoming 2013 SR1 release will allow installation without any additional steps.