Solution:

Most magnets have their North pole on the outside. There are however only 17 faces with a South pole. It just so happens that the 3 by 4 rectangle has exactly 17 touching faces, so every South pole must be used up by some adjacent pair.

There are four cubes that have only North pole magnets. These cubes can be easily recognised by testing all its faces against the face of another cube. All its magnets will behave the same way (all attract, or all repel). Once you have found one such all-North cube, you can use it to test all the faces of all the other cubes. You should find the following arrangements:

Amount	Picture	Description
4		No South poles
2		One South pole
1		Two South poles, on opposing faces
2		Two South poles, on adjacent faces
3		Three South poles

There are 156 solutions. Below is a complete list of them.

1	2	3	4	5	6
7	8	9	10	11	12
13	14	15	16	17	18
19	20	21	22	23	24
25	26	27	28	29	30
31	32	33	34	35	36
37	38	39	40	41	42
43	44	45	46	47	48
49	50	51	52	53	54
55	56	57	58	59	60
61	62	63	64	65	66
67	68	69	70	71	72
73	74	75	76	77	78
79	80	81	82	83	84
85	86	87	88	89	90
91	92	93	94	95	96
97	98	99	100	101	102
103	104	105	106	107	108
109	110	111	112	113	114
115	116	117	118	119	120
121	122	123	124	125	126
127	128	129	130	131	132
133	134	135	136	137	138
139	140	141	142	143	144
145	146	147	148	149	150
151	152	153	154	155	156