From the Humminbird site:
Can I convert my Lowrance or Garmin Waypoints with Humminbird PC 3.026 or higher using an MMC/SD card?
To see a video of this operation click here and then click on FAQs.
When performing the following task the user must have the Lowrance or Garmin information saved on an MMC/SD card. The user must also have the card placed in a card reader connected to the computer.
Open HumminbirdPC
Click on File then OPEN
The program is now going to ask what to import and where it should look for the data. In the LOOK IN section in the top window browse with the drop down box to select the MMC/SD card (the MMC/SD drive may also be listed as a removable drive).
Then go down to the bottom box - FILES OF TYPE - click on the the down arrow it will give a list of all recognizable format. Select the correct format for the file you want converted. Lowrance is a .USR or a . LOW file. Garmin files would be a .GDB or a .MPS..
Specifiy which waypoint files you would like to switch to the Humminbird format and the files will be converted automatically. The files will then appear in the left hand column as a .gpx file. It will have the same name as the orginal file with the .gpx extension behind it..
You would then use then hightlight the new .gpx file and select the the green up arrow under the image of an SD card to install the file to the SD card. You will get a message to ADD or REPLACE data on the card. Select ADD, unless you want to delete everything else on the card then you may select REPLACE. The new gps data will then be loaded to the card. You will see a progress bar showing the files being uploaded to the card.
Once complete you will then press the RED up arrow under the SD card Icon on the Humminbird PC program and then when it is safe to remove the card from the card reader the SD icon will disappear from the Humminbird PC program. You may then safely remove the card from the card reader.
Plug the card into the Humminbird unit while it is powered on and the information will then automatically load onto the unit. It will tell you on the unit that it is Loading Waypoints, Routes, and Tracks and then state when it is complete and how many routes, waypoints and/or tracks was loaded into the unit..