Match scans your iTunes library and your ext hdd must be attached to your computer. However, if it is your intention to stream music from your computer, you should try creating a new blank library and then sign into the store and activate match. This will now show all music you have in match and the number of tracks should be same as shown on your ipad. All tracks will have download cloud icon.
To add tracks or update match go back into your original library. You should add iCloud status column to your song view. This will tell you if the track has been matched, uploaded, purchased, ineligible or waiting.