  We compare the magnetic and electronic configuration of single Mn atoms in
molecular beam epitaxy (MBE) grown Bi$_2$Se$_3$ thin films, focusing on
electron paramagnetic (ferromagnetic) resonance (EPR and FMR, respectively) and
superconducting quantum interference device (SQUID) techniques. X-ray
diffraction (XRD) and electron backscatter diffraction (EBSD) reveal the
expected increase of disorder with increasing concentration of magnetic guest
atoms, however, Kikuchi patterns show that disorder consists majorly of
mum-scale 60deg twin domains in the hexagonal Bi$_2$Se$_3$ structure, which are
promoted by the presence of single unclustered Mn impurities. Ferromagnetism
below T$_C$ ~ (5.4 +/- 0.3) K can be well described by critical scaling laws
M(T) ~ (1-T/T$_C$)$^\beta$ with a critical exponent $\beta$ = (0.34 +/- 0.2)),
suggesting 3D Heisenberg class magnetism instead of e.g. 2D-type coupling
between Mn-spins in van der Waals gap sites. From EPR hyperfine structure data
we determine a Mn$^{2+}$ (d$^5$, S = 5/2) electronic configuration with a
g-factor of 2.002 for -1/2 --> +1/2 transitions. In addition, from the strong
dependence of the low temperature FMR fields and linewidth on the field
strength and orientation with respect to the Bi$_2$Se$_3$ (0001) plane, we
derive magnetic anisotropy energies of up to K1 = -3720 erg/cm3 in MBE-grown
Mn-doped Bi$_2$Se$_3$, reflecting the first order magneto-crystalline
anisotropy of an in-plane magnetic easy plane in a hexagonal (0001) crystal
symmetry. Across the ferromagnetic-paramagnetic transition the FMR intensity is
suppressed and resonance fields converge the paramagnetic limit of a Mn$^{2+}$
(d$^5$, S = 5/2).
