The slot method of loading maps through the default multi-player map menu is the old way of loading H2.5 maps, the new way is through the custom map menu.
Also make sure you do not have more then 48 maps here:
C:\Users\USERNAME\Documents\My Games\Halo 2\Maps
The slot method is now used for co-op campaign, extra MP map storage, and further restricted content testing. Yes, you need to know what slot is what but again this is more for developers/map makers then players. So you should not be dealing with slot maps at this point.
They (Microsoft) removed the feature to add a custom map picture in one of the few game patches.
Also make sure you do not have more then 48 maps here:
C:\Users\USERNAME\Documents\My Games\Halo 2\Maps
The slot method is now used for co-op campaign, extra MP map storage, and further restricted content testing. Yes, you need to know what slot is what but again this is more for developers/map makers then players. So you should not be dealing with slot maps at this point.
They (Microsoft) removed the feature to add a custom map picture in one of the few game patches.