Manganese has five unpaired electrons, but Iron has four, then why is Iron ferromagnetic and Manganese paramagnetic? What's that property I'm missing?
I'm guessing you meant Manganese rather than Magnesium, since Manganese has five unpaired electrons but Magnesium doesn't.
The answer is that ferromagnetism is not simply a function of having unpaired electrons. The effect is far more subtle than that. You would expect that the unpaired electrons in any material would tend to align themselves in opposition to each other because this is usually the lowest energy configuration. However in addition to the usual charge and magnetic interactions there is an interaction called the exchange interaction (I've linked to the Wikipedia article, but this is a complex area and hard going for the beginner).
Whether a material is ferromagnetic depends on the relative strengths of the exchange interaction with the other interactions, and it's a fine balance. This means even small changes may change a material to ferromagnetic or back. For example, although Iron is the best know ferromagnet not all crystal forms of Iron are ferromagnetic. The austenitic form of Iron is paramagnetic not ferromagnetic, so just changing the crystal structure slightly can switch between ferromagnetism and paramagnetism.
In Manganese the balance of the interactions prevents ferromagnetism, though note Manganese alloys like Heusler alloy can be ferromagnetic.
Atoms or molecules of paramagnetic materials (Magnesium) have permanent magnetic moments (dipoles), even in the absence of an applied field. The permanent moment generally is due to the spin of unpaired electrons in atomic or molecular electron orbitals (see Magnetic moment). In pure paramagnetism, the dipoles do not interact with one another and are randomly oriented in the absence of an external field due to thermal agitation, resulting in zero net magnetic moment. When a magnetic field is applied, the dipoles will tend to align with the applied field, resulting in a net magnetic moment in the direction of the applied field. In the classical description, this alignment can be understood to occur due to a torque being provided on the magnetic moments by an applied field, which tries to align the dipoles parallel to the applied field. If there is sufficient energy exchange between neighbouring dipoles they will interact, and may spontaneously align and form magnetic domains, resulting in ferromagnetism (Iron). It doesn't depend on the number of unpaired electrons available for its nature but on the amount of energy required to form domains in the atom.
Ferromagnetism is the characteristic for magnetically concentrated materials; i.e, the unpaired-electrons containing atoms are very close to each other, this creates domains in the crystal of unpaired electrons that have the same direction by supporting each other. So a large magnetic moment (field) in the same direction and as a result permanent magnet unless heat is supplied to overcome the cooperative interactions between the small magnets.
However, paramagnetism is characteristic for magnetically dilute matter such as unpaired-electrons containing centers are separated from each other in large molecules. The magnetic moments of such magnetic atoms are oriented randomly and their magnetic moments cancel out each other and no permanent magnet. But by applying an external magnetic field the magnetic moments are oriented in the same direction with the applied field so the outcome field from the material is larger than the applied.