the last time I checked, this is the way it's supposed to work ...
Structured Text is "probably" already installed - but it will NOT function unless you have purchased and activated a license for that particular option ...
contact your local Allen-Bradley distributor - and have your software's serial number handy ... they will "probably" give you a quote for the extra function ... you'll pay the price - and then you'll be allowed to download the license over the Internet ... once that's done, the extra ST function should work ...
TIP: if you think that you might need additional "extra features" in the future, (example: Function Block Diagrams, Sequential Function Charts, etc.) then you should ask about a price for a "bundle" which would include them all ... this is usually cheaper than adding the options one by one ...