My update for anyone else having this problem as well. You may want to read the detailed explanation here
ExpressionEditor already contains an expression ready to be compiled. To avoid creating strongly typed expressions (in many complex scenarios it is impossible to know the type of result value) you can use "dynamic" objects (DLR that is part of Silverlight + Microsoft.CSharp.dll reference) to compile and execute expression.
Here's a quick usage sample:
dynamic dynamicExpression = expressionEditor.Expression;
dynamic compiledExpression = dynamicExpression.Compile();
object executionResult = compiledExpression(myInstance);
Hope that helps,