It's easy! To add entries manually, you'll go to your giveaway's entries tab, click on the 'show me the entries' button to open the grid, and add entries on the top left of the grid that appears.
Here are a couple things to note about this feature:
#1 - When adding a manual entry, you must enter a name, email address, and an entry option for it to save. If you are using the free subscription or Basic subscription of Rafflecopter, you must select an entry option besides the "Referred a Friend" entry option for it to save.
If you have a daily entry option, you can only add one manual entry per 24 hours / day. Meaning if you wanted to add 5 manual entries for someone that had wanted to enter a daily entry option 5x, you'd have to add them over the course of 5 days, one per day.
#2 - The "View Entries" table shows between 250-5000 rows per page. By default it only shows 250. If you have a large giveaway, make sure you view the last page after adding a manual entry, or sort by "entry # descending", or use one of the filters to confirm your manual entry worked. When typing into a filter field, make sure to hit your 'enter' key to filter on the entire dataset.