Thanks Walter again, The post itself was not very enlightening to me although it gave me some guidelines. Actually there was a link in it to the red giant website. I checked it and then I found out where the media core folder was. So this was the simple solution for me that worked:
1) duplicate the whole content from of AE CS5’s media core folder to the mediacore folder of AE CS5.5
(from hd/library/applicationsupport/Adobe/common/plug-ins/CS5/mediacore
to hd/library/applicationsupport/Adobe/common/plug-ins/CS5.5/mediacore)
2) Duplicate the missing plugs in AE CS5 effects folder to AE CS5 effects CS5.5 folder
3) Duplicate presets and scripts folder in the same manner
AE CS5.5 starts up smoothly, no message error, all the effects from CS5 projects are made available in new project. Except for the FxFactory plugs but this xas because my version is too old.