In order for M-Power to export sales (invoices) or bills (purchases) each line that is exported requires an Item ID to be selected so that the line is correctly recorded in Zero.
To create the items in Xero follow this process:
1. Select Settings  from the main Xero menu then Inventory Items .
2. Click the New Item  button.
3. In the New Inventory Item dialog enter the Item Code  and Item Name . Then fill out the details for either or both the purchase and sell sections depending on the item you are creating. Note that you should check with your accountant to determine the correct accounts [4 & 6] to choose to ensure that the invoices and bills are correctly applied to your chart of accounts. Ensure that you enter a Purchase and/or Sales Description (in addition to the Item Name). If the description field is blank M-Power will not allow selection of that item in the invoice/bill screens.
Finally click save to save your changes.
4. Once you have added the item to Xero you will need to refresh M-Power's copy of the Xero Data.
To do this when invoicing go to an invoice and select the Other tab , then click Refresh 
To do this when exporting bills go to a bill and select Refresh Lists 
If for any reason your item is still not visible then exit and re-enter the invoice or bill.