See the NSString +stringWithFormat: documentation.
In particular, look at the "Formatting String Objects" examples and the "String Format Specifiers" documentation.
You either need to change your format string to:
NSString *searchURL = [NSString stringWithFormat:@"https://www.google.com.au/webhp?sourceid=chrome-instant&ion=1&ie=UTF-8#hl=en&safe=off&tbo=d&sclient=psy-ab&q=%@", [_textbox1 stringValue]];
Or you could just say:
NSString *searchURL = @"https://www.google.com.au/webhp?sourceid=chrome-instant&ion=1&ie=UTF-8#hl=en&safe=off&tbo=d&sclient=psy-ab&q=";
searchURL = [searchURL stringByAppendingString: [_textbox1 stringValue]];