Bonn,
I am afraid, I am not able to replicate your issue at my end. I have added CheckStyle to ME 8.6 without any issue. Here is what I did –
1. From MyEclipse IDE, click on Help > MyEclipse Configuration Center
2. Choose Software tab. Search checkstyle in the search plug-ins textbox
3. It lists various options, I choose “CheckStyle Plugin” under Popular plugins.
4. Right click on the same and click on [Add to profile]
5. Click on [Apply 1 change] under *Pending Changes*
6. This opens a wizard which guides you through the rest of the steps
7. When prompted, please choose to restart the IDE
Let me know how this works for you.
Could you please re-try the above steps. In case this is still and issue, please provide below details –
1. From MyEclipse IDE menu options click on MyEclipse > Installation Summary > Installation Details and copy and paste the same here
2. Attach a screenshot of the issue
3. Copy the whole error and copy and paste it here
4. Are you behind firewall / proxy? If yes, please setup your proxy here – window > preferences > general > network connections
5. Clear your .log file located at [your workspace dir]/.metadata/.log, replicate issue and attach the same here