OOPS!
Forgot MyRS.MoveNext in the Do While loop and the
quotes in MySql were wrong. This is the corrected code:

cValueList = ""
do while not MyRs.EOF
  cValueList = cValueList + MyRS.Fields("<field_name_here>").value + ","
  <b>MyRS.MoveNext</b> 
enddo

* strip the last comma and add quote
cValueList = left(cValueList, len(cValueList)-1)

* (you may need to enclose values in quotes, depending the column types being integers or strings)

MySql = "Select * From MyTable Where IdNo IN (" + cValueList + ")"
nResult = SqlExec(nHandle, MySql)

HTH