Something like
ConditionExpression condition1 = new Microsoft.Xrm.Sdk.Query.ConditionExpression
{
AttributeName = "statecode",
Operator = ConditionOperator.Equal,
Values = { "Active" } //or SomeEnum.Active.ToString() if you want use good practice
};
should work fine.
Edit:
other option is to cast enums to int: Values = {(int)SomeEnum.Active}