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You've got some Typos in there and setting a multivalue list is done a little differently. Switching up your code to something like this should get you going:

 

Dim doc As AssemblyDocument = ThisDoc.Document
Dim oAssDef As AssemblyComponentDefinition = doc.ComponentDefinition
Dim oUsPar As UserParameters = oAssDef.Parameters.UserParameters
Dim oPar As UserParameter = oUsPar.AddByValue("Side", "", UnitsTypeEnum.kTextUnits)

Dim oArr As ArrayList = MultiValue.List("Side")
oArr.Add("Left Side")
oArr.Add("Right Side")
MultiValue.SetList("Side", oArr.ToArray())