Baking a cake, while not a monumental undertaking, still takes a bit of time out of your day, and when it doesn't go well, the results can be disappointing. Making a cake that calls for dried fruit such as raisins in the batter can be even more challenging since the raisins often sink to the bottom of the batter before the cake bakes completely. It is possible to keep them well mixed throughout the batter, but you have to pay attention to the consistency of the batter.
Chop raisins into small pieces before putting them in your batter. This will make them lighter overall and keep them from sinking, especially if you are using plain flour (which is finer than all-purpose flour).
Make your cake with all-purpose flour in your recipe instead of plain flour. All-purpose flour is not as finely ground as plain flour and holds ingredients in the batter up a little better.
Toss your raisins in flour or in the mixture of dry ingredients before putting them in the batter. This helps the raisins hold their place in the batter better than if they are thrown in uncoated.