Go to your forum and look at the bottom where it says Feeds, click on that link and then two links of the Feed will appear (the first being the Topics only Feed and the other one is Topics and replies) - choose the one you want.
After this, log in to your Blogger Dashboard, go to Layout, click on Add a Gadget link, choose Feed from the gadgets list and paste the Feed URL you have selected inside the text box.
Click on Continue, and you only have to configure the parameters, then Save changes to see the results.
That's it! Enjoy :)