In your CONTROL FLOW tab, select the data flow step that contains the XML Source step. Do not go into the dataflow, it must be done in control flow.
1. Bring up its properties (right-click > Properties)
2. Click the elipsis button next to EXPRESSIONS
3. Select the [XML Source].[XMLSchemaDefinition] property then hit the elipsis button to the right of the Expression textbox to set the XSD File path
4. Drag and Drop your variable that holds your XSD path into the expression box, you should be able to evaluate expression here.
5. Click OK
6. Click OK again