To load the MIB into PRTG you must go through the process of converting it to a PRTG compatible .oidlib file. This is done with the Paessler MIB Importer. From there, you can import the MIB/TXT file provided with the firmware release, make any necessary changes and then save this in your PRTG SNMP Library folder.
To save you a step, we have provided PRTG compatible OID Library files below. Simply save these in your "snmplibs" folder in your PRTG installation directory. Further information is provided by Paessler on the page linked above.
You may also want to see this article for a suggestion on items to monitor: Items to Monitor with SNMP
Note: Starting with the 3.0 release for GigaPlus, PTP products will being to use the same MIB so a separate .oidlib file will not be necessary. I.e. with GigaPlus 3.0 or ApexPlus 3.0, please use the TL-PTP-v3.oidlib file. For previous releases and products you'll need to use the specialized file.