How to correctly wire this transformer for 240V?
The primary windings go in series, but get it wrong and the transformer turns into a (bifilar wound) resistor. It is important to not connect them in "anti-series". It cannot be seen from the outside how the windings are internally connected. You 'll have to wire it, then carefully test if the transformation ratio primary-secondary is about right.

I think it'd be connected as below, but you'll have to carefully verify the result before using it at full power. 

As @jms suggests, add a (regular old fashioned incandescent) 240V light bulb in series when testing. If it lights up while the transformer is unloaded, then you've wired it incorrectly.

~ ---o   0  o
           /
          /
         /
        /
       /
      /
     o  120 o--- ~


I understand from your comments that the transformer is rated for 240V, but do be careful, a new transformer with decent spec sheet is cheaper than rebuilding a burned down lab.