First of all, I know that “MBP” stands for “MacBook Pro”, but what does “MM” stand for?
Second of all, what version of Mac OS X is your MacBook Pro running? (Click on “About This Mac” under the Apple Menu.) If it is running Lion (10.7.x), then you should probably rewrite the script for your “MM”, unless you already did that.
Third of all, is access to assistive devices enabled on your “MM”? If not, then this may be your problem. Open System Preferences and click “Universal Access”, then click the check box that says “Enable access for assistive devices”. (You will need to type an administrator password.)
tell application "Safari"
activate
delay 4
tell application "System Events" to tell process "Safari"
keystroke "http://192.168.1.90/37"
end tell
end tell
Although listed in System Events’s Processes Suite, ‘keystroke’ and ‘key code’ don’t require access for assistive devices to be enabled. Nor do they directly access any part of a process’s GUI, so it makes no difference whether a script includes a reference to a process or not. Keystrokes and key codes are simply generated and land on the frontmost process. What matters is that the target process is frontmost and that its target field is focussed.