In iMovie's menu select Edit>Special Characters. A window will open like the following:

Click on the item Letterlike Symbols. To the right you will see all the various character options, including the ™ symbol. Double click on the symbol to add to your text at the cursor point.
For ease of access, you can include the Keyboard and Character Viewer in the Finder Menu Bar. Open System Preferences and select Language & Text under the Personal heading. Under the Input Sources tab, check (tick) the boxes labelled "Keyboard & Character Viewer" and "Show Input Menu in Menu Bar". See the following graphic:

John