Adding it to Ultimate, provided that you haven't added other mods that take up block ids:
1) Naviagate to the millenaire folder and find config.txt.
2) Open it in Wordpad (not notepad, which will make it obnoxious to read)
3) Scroll down and find the line that says "//IDs of the Millénaire blocks". Change that and the following respective lines to this:
//IDs of the Millénaire blocks
//New ids for 3.0
block_building_id=2275
block_panel_id=2276
block_wood_id=2277
block_earth_id=2278
block_stone_id=2279
block_crops_id=2280
block_panes_id=2281
block_byzantine_brick_id=2282
block_byzantine_slab_id=2283
block_byzantine_mixedbrick_id=2284
//ID of the first Millénaire item (the others are in sequence after it)
//New ids for 3.0
item_range_start=2285
Ignore the apparent "second set" of ID assignments. You will note those lines begin with "//", which means they are commented out and are not doing anything.
4) Save config.txt
5) Add Millenaire as usual through the "edit mod pack" button in the FTB launcher.
...of course many other ID ranges would work, but if you're totally unfamiliar, this will do. Basically you only have to do this because the default ID range (I believe 1515 and on...) conflicts with some of the block IDs taken up by Twilight Forest.