Download and extract the file and then open it with a text editor to see the list of rules, do you know which rule you are looking for?
To import a SNORT rule you use the 'Custom Attack Editor' feature located at Policy > Advanced > Custom Attacks > Custom Attack Editor
Launch the 'Custom Attack Editor' and go to Attack > New > SNORT Rule.
Past the rule you want to add into the window and press save.
Goto File > Save and save the update
I assume the 29909 rule is not in the above file as you need to pay / subscribe to get it.