In this case I do not think the file choose function could work since it would supposedly prompt you to enter a file path and this does not work from within the node.
You would have to enter the path manually or use a string widget node.
knime.out is the data frame that will be read back to KNIME so your imported file will have to be assigned there
knime.out <- dataset
An example how to bring have paths and R in KNIME work together can be found here