Hey ionnek. :)
1) Start Synaptic
2) Go to Preferences->Repositories in the synaptic menu
3) Find the line that says http://sluglug.ucsc.edu/macromedia/apt/
(or any line with "macromedia")
4) Put a check next to that line, so it's enabled and hit "OK"
5) Click "Update List" button in synaptic. This will download the list of available applications.
6) Then find "flash-plugin" in the list and highlight it. Click the "install" button at the bottom right.
7) Then hit "execute" button at the top
8) This will then tell you what it's gonna do--acknowledge it by hitting "proceed"
Different versions of synaptic may be slightly different.