I can't get the SKU value for a commerce product in a rule action
In order to access the SKU for the product you must first use two conditions in your rule to assure that you are dealing with the right type, these are : 


Entity is of type commerce-line-item
Entity has field commerce_product


Just as an example this Rule export applies a discount ( 5_eur_disc ) to a product with an SKU of SKU-FOO-ITEM after checking that the discount dates are current.

{ "commerce_discount_rule_discount_discount_per_sku" : {
    "LABEL" : "Discount per SKU",
    "PLUGIN" : "reaction rule",
    "REQUIRES" : [
      "commerce_discount_date",
      "rules",
      "commerce_discount",
      "commerce_product_reference"
    ],
    "ON" : [ "commerce_product_calculate_sell_price" ],
    "IF" : [
      { "commerce_discount_date_condition" : { "commerce_discount" : "5_eur_discount" } },
      { "entity_is_of_type" : { "entity" : [ "commerce-line-item" ], "type" : "commerce_line_item" } },
      { "entity_has_field" : { "entity" : [ "commerce-line-item" ], "field" : "commerce_product" } },
      { "data_is" : {
          "data" : [ "commerce-line-item:commerce-product:sku" ],
          "value" : "SKU-FOO-ITEM"
        }
      }
    ],
    "DO" : [
      { "commerce_discount_fixed_amount" : {
          "entity" : [ "commerce_line_item" ],
          "commerce_discount" : "5_eur_discount"
        }
      }
    ]
  }
}