You have 3 options:
1. Reinstall StarCraft
2. Wait for the next version of StarForge
3. Change the registry settings manually
I'll walk you through option number 3:
1. Open the start menu and select "run." Type "regedit" and hit enter.
2. Find HKEY_LOCAL_MACHINE -> Software -> Blizzard Entertainment -> StarCraft. Be sure to click on the StarCraft folder. Several items should appear in the right-hand pane.
3. Double-click the item named "InstallPath." Change this to your actual StarCraft directory. For example, mine (and the default) is "C:\Program Files\StarCraft"
StarForge should work after completing this.