In the bugs tab in Milestones, click Link Bugs. A pop up will appear to show a list of available bugs on the right side. Click on the bug to add them to the right side, and click Add.
Users will need view permissions in Bugs to link an existing bug a to milestone. Additionally, depending on a user's project permission settings, they may be able to see all the project bugs or just a subset of them. This is true for the bugs listed in the milestone's bug tab, and the list of existing bugs when linking a bug to a milestone.