If you wish to check out the procedure just follow what I have done
step1: import module from marketplace. and create a entity based on the excel label/title[eg: firstname,lastname,country]
step2: Navigation the Mx object overview and Excelimport overview.
step3: Select the module entity that you created in Mxobject overview page and refresh the entity [Synchronize all entities and microflows].
step4: Now go to Excelimport overview and create a template, in mendix object select the created entity and click save and generate column. Import action set as synchronize objects.
step5: arrange the attributes based on column number in excel.
step6: In tab container go to import file page → select the new button → select created template → and select the excel file [note:it should be in .xlsx(excel workbook) format].
step7: And select the created one and press import file , now your records are imported.
The noclass found error points to some clashing userlibs. Double check your userlib directory for double JAR files. The likely cullprit would be the org.apache.commons.lang3.jar where you have multiple copies of.