to piggyback on this line of thought, there is something else you need to consider (actually Several things)
1) Tech Limits: Introductory, Standard, Advanced, Experimental, Unofficial
When you start a campaign using Megemek HQ, under Campaign Options's Tech Limits Tab, you get to chose what technology limit you wish to operate under. In effect, you are deciding if you want to play (in terms of what technology is available to you to purchase or what the OPFOR will also have). This is important because you will ONLY SEE what the Tech Limit allows in terms of parts availability, what designs can be seen in the Auction House, etc. That distinction is also important as explained next...
2) When you create custom designs (in whatever program you used i.e. Megamek, Megamek Lab or Megemek HQ under the Mek Lab tab), the Tech limit is what you select it to be. Why is it important? Because if you make a design with it's own Tech limit set to Unofficial, if any of your IG settings are NOT set to Unofficial, you won't see it as an option to refit it later (Its not that its not there, it's the settings PREVENTING IT from being available or even viewable) or able to buy it in the AH*
* Assumes that you selected that option to allow such purchases in Campaign Options
3) Irregardless of above, there is one last thing you need to do (and it applies to doing it in all 3 megamek games individually): There is a file, called Units, in the Data/Mechfiles folder. That file is what the game reads to make viewable any official or custom files IG. In order to make any customs you make, viewable, first, Close out or Save and Close out what ever MM program you used to create your custom design. Next you need to DELETE that file (and only that file). Don't worry: When you restart which ever MM program you used, the game will automatically create a replacement Units file after it has spent a moment looking for all the it recognizes as MM units (it will look like its lagging a moment, that's fine, its looking within its file portion for ANY MM that corresponds with any unit IG.
Thats another common mistake in why you sometimes don't see particular or non-cannon designs you made.
I'm sorry that i made this very wordy but i hope you understand my point, sir...