Certain steps (for example the 'Get File Names') require wildcard file names. You sould supply a regex for the file names, for example ".*\.csv$"
The Properties input step is able to resolve variables within the file before loading the data onto the stream. For example, if you have the following lines in your properties file:
ROOT_DIR=/my/dir/
LOG_DIR=${ROOT_DIR}/logs
you can resolve LOG_DIR as /my/dir/logs by setting the 'Substitute variable for value' checkbox in the Content tab of the Properties File dialogue.