Bug 37353 add variable button can't add variable only edit. [build path}
Project->properties->java build path->Library tab->add variable button will only allow you to extend or edit a variable not add a variable. The only way to add a classpath variable is to go Window->preferences->java->classpath variables->new button.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/NewVariableEntryDialog.java
List selected= fVariablesList.getSelectedElements();
		int nSelected= selected.size();
		
		if (nSelected > 0) {
			fResultPaths= new Path[nSelected];
			for (int i= 0; i < nSelected; i++) {
				CPVariableElement curr= (CPVariableElement) selected.get(i);
				fResultPaths[i]= new Path(curr.getName());
				if (!curr.getPath().toFile().isFile()) {
					status.setInfo(NewWizardMessages.getString("NewVariableEntryDialog.info.isfolder")); 
					canExtend= true;
				}
			}
		} else {
			isValidSelection= false;
			status.setInfo(NewWizardMessages.getString("NewVariableEntryDialog.info.noselection")); 
		}
		if (isValidSelection && nSelected > 1) {
			String str= NewWizardMessages.getFormattedString("NewVariableEntryDialog.info.selected", String.valueOf(nSelected)); 
			status.setInfo(str);
		}
		fCanExtend= nSelected == 1 && canExtend;
		fVariablesList.enableButton(0, fCanExtend);
		
		updateStatus(status);
		fIsValidSelection= isValidSelection;
		Button okButton= getButton(IDialogConstants.OK_ID);
		if (okButton != null  && !okButton.isDisposed()) {
			okButton.setEnabled(isValidSelection);