This is derived from a neuter Latin word, so you have two alternatives. The Latin plural is practica, which is also correct in English. The Anglicized plural is practicums, which was used on dictionary.com as a sample sentence for the word. Many Latin words that are commonly used in English will often be pluralized with -s or -es, instead of their original Latin plural. If you are ever in doubt, the Latin plural is always correct.
Some other examples where both plurals are given in the dictionary:
Genus - genera or genuses
Antenna - antennae or antennas
octopus - octopi or octopuses
Radius - radii or radiuses