First of all, 3dLME should automatically generate an F-statistic for the interaction between the two factors. If you want to pinpoint anything further about the interaction, you have to be more specific about exactly what kind of contrasts you're looking for. For example, you could run a few contrasts such as ValencePositiveArousalLow versus ValencePositiveArousalHigh, ValencePositiveArousalLow versus ValenceNegativeArousalLow, etc.
Gang