Dear Aitor,
In Modes, The total beam length should be the height of the tower or the tip radius of the blade (TowerHt for the tower or TipRad for the blade in FAST v8’s ElastoDyn module) and the rigid beam length should be the rigid portion of the bottom of the tower (TowerBsHt, if any) or the hub radius of the blade (HubRad). End mass should be the total mass of the tower top / rotor-nacelle assembly for the tower (found in the ElastoDyn summary file) or the tip mass for the blade (TipMass).
Modes cannot be used to model an embedded tower with flexible foundation. For this, you should use BModes in place of Modes. In fact, as discussed in my May 31, 2012 post in the following forum topic: Tower Eigenfrequencies of NREL 5MW Turbine, we now recommend that BModes be used in place of Modes in all cases (but especially for the tower).
Best regards,