I'm guessing it's this bit of code:
& "<B>Visitor Name: </B>" & +EmployeetoMeet.ToString() + +"<BR>" _ & "<B>Visitor Contact No: </B>" & +EmpMobileNo.ToString() + "<BR>" _
Sometimes you are using & to concat but you have a bunch of + signs in there....it's probably trying to convert the string to a double because it thinks you want to perform a mathematical addition. Get rid of the + signs before
EmployeetoMeet.ToString()
and convert the + signs after into a single &.