On my HUD there is a tower marker behind the Mass Fusion building that I can't seem to activate. I'm assuming it is the roof of the Mass Fusion building but I've been all over the inside of that place and can't find a way up there. A person in another forum mentioned something about being able to activate it by "...getting on the bridge from ground level then going through an adjacent building then climbing steps on the side of the building where the waypoint is." I have no idea what he was talking about, anyone have an idea what that is?

What compass direction from the Mass Fusion building marker is it? I'll take a look in the official guide and see if I can't help out. No guarantees, but I'll see what I can do.

EDIT: Just looked up the location in the wiki, and the map shows an identical icon NNW of the Mass Fusion building. The best guess I have is that it's not the roof of the Mass Fusion building, but the Boston Bugle building, which would be a separate location, I would think.

What quest are you doing? Roof is accessible, but it may depend on the quest you are currently doing.

Just took another look at the wiki, this time at the Boston Bugle Building entry. The icon NNW of the Mass Fusion building is for the Boston Bugle, not for the roof of the Mass Fusion building.

Don't want to spoil anything but I don't think you're supposed to be able to reach the roof yet during the mass fusion quest.

I'm not anywhere near my console at the moment so I can't say what direction it is, but if you are staring at the front doors of the Mass Fusion building then it is right behind it. I circled the building and the icon appeared to be inside it but I couldn't activate it. Not on any particular quest either I just like activating the icons as I come across them.

The mass fusion in downtown area? I have been teleport there after blow the institute with the militia and the card was there in à desk

That's what I was trying not to spoil. Though I realize this is the spoiler section. ..

So I have to progress in the main story to get there then?

Maybe, than i guest you need to blow shaun to get there, oh by the way there not a lot there nothing special

I'm going Minutemen on this playthrough so I hope that doesn't screw me over.

Turns out that there's a map marker called 'Mass Fusion Executive Suite' just East of the Mass Fusion Building marker. The official guide doesn't have a marker listed for this, and the wiki doesn't have any information on it, but the Mass Fusion Building entry mentions that you can arrive on the roof while a quest is active. Looking at the appropriate quests mentioned in the entry, you'll end up on the roof by siding with either The Institute or the Brotherhood of Steel. Siding with the Railroad or the Minutemen, on the other hand may render this map marker undiscoverable, as these two factions don't have quest hooks at the Mass Fusion Building.

That roof area is not really accessable from any nearby area.
Even a jetpack wont be able to lift you there.

Once one has been there, it does have its own fast travel marker.

The only other possible way there may be using the "land here" option on a vertibird, if you are feeling lucky.

I landed on the deck of the USS Constitution that way once.
Selected land here as some idiots on the ground were firing on us.