Hi Javier,
I have double checked the function. It properly returns a node number. I think your problem is the coordinate system. FEFLOW works with the local coordinate system, thus you will need to subtract the origin (X,Y) to the coordinates before you pass these values to the IfmFindNodeAtXY function.
node = IfmFindNodeAtXY(m_pDoc, 352, 1715, pointer1);
IfmInfo(m_pDoc, "Node: %i", node);
The second statement will print the number on the Log panel. After FEFLOW 6.2 (p12), we introduced two more functions for searching nodes or elements:
int (*findElementAtXYZ) (IfmHandle, double x, double y, double z);
int (*findNodeAtXYZ) (IfmHandle, double x, double y, double z, double* dist);
If you need just to know the number of the node and/or element, you can simply create a nodal/elemental expression distribution in FEFLOW GUI. Remember to decrease one unit to the number you get (to be in concordance with C++ counts from zero).
Cheers,
Carlos