Can you show us the error report? It looks like thats how you install it, and the problem is just you need to change the ids for some of the items and blocks. To do that, go into the FTB Monster folder. There should be a file called IDConflcits. It should contain a list of conflicting ids and at the bottom it has suggestions for what ids to use. You need to go into the config folder and find the config for Millenaire, then open the config and find the values that are listed in the IDConflicts file. Then change these numbers to one of the ones that the IDConclifcts file advises you to use. After that run it again. You may have to repeat this a few times.
If theres no file called IDConflicts, tell me, as theres a way to do it without the IDConflicts file.