The only pronunciation recorded in OED (1989 edition) of the adjective electoral is with the stress on the second syllable, 'e-LECT-er-el' (IPA: /ə (or ɪ) ˈlɛkt ə rəl/). LPD records the marginal variant 'e-LECT'rel' (IPA: /ə (or ɪ) ˈlɛk trəl/). This, then, is the traditional British pronunciation.
Recently (2010) another pronunciation, marked by LPD as 'non-RP', has been gaining ground. This has the stress on the third syllable, 'e-lect-OH-rel', IPA: /ə (or ɪ) lɛkt ˈəʊ rəl/. This seems unnecessary, but students of such subjects as Politics should learn what their teachers expect. Others may hardly ever use the word. If you should be in doubt, AWE's advice is to use the traditional pronunciation - the first one listed above.