I tested this with FMP6, so this may (or may not) work for you:
Create a new text calculation field, naming it 'ct_imdb'
Use this as the calculation, substituting your field names for the actor's first and last name: "http://www.imdb.com/find?s=all&q=" & first_name & "+" & last_name
Make sure to select Text as the format
Create a new script, naming it IMDB
Add the "Open URL" script step, check the Perform without dialog box, then click the Specify... button
Choose Field Value, select the 'ct_imdb' calculation field
Click OK, close the script, add a button on the layout, set to Perform Script, selecting the IMDB script