In this context Experten is nominative plural, but it is not always the case. Der Experte (http://dict.leo.org/ende/index_de.html#/search=experten&searchLoc=0&resultOrder=basic&multiwordShowSingle=on) is a "weak" or "n" declension noun (http://german.about.com/od/grammar/a/dernouns.htm). This noun will take an -en in all other cases besides nominative (which is why we know this is plural, because if it was singluar it would be "Hallo Experte"). So keep that dictionary, it is leading you on the right path because "Experten" can be either singluar or plural depending on the context (ie. word order, article, etc.). So for example mit dem Experten = singluar but mit den Experten is plural.