However, since you mention that several OXPs don't appear, it may be that the OXPs in question were not correctly installed.
OXP folder-names must end with ".oxp" for Oolite to recognise them. Most OXPs will, when unzipped, be in a folder that does not have this extension. If you look inside this folder, you will see a folder with the '.oxp' extension. That is the folder you must copy to the AddOns folder. If you have simply unzipped the OXP into the AddOns folder, it's not going to work.
If this isn't enough to solve the problem, post a list of the OXPs you tried to install, and the contents of the 'Latest.log' file, and we'll do what we can to sort it out. The location of the log varies, depending on which Operating System you are using. From the relevant part of the Oolite FAQ on the Wiki:
Incidentally, you need to be aware that any missiles, bombs or mines you add to the game, can also be used against you by the bad guys.. (fortunately, in the case of the I-Missile, the chance is only 1 in 1000) so don't be in too much of a hurry to add uber-weapons to your Ooniverse before you're able to cope with them.Mac OS X: ~/Library/Logs/Oolite/Latest.log (that is, in your home folder, open the folder named Library, then Logs, then Oolite). Alternatively, select “Show Previous Log” from the “Oolite” menu in the menu bar when running Oolite.
Windows: in Oolite’s folder (typically in C:\Oolite), there is a folder called oolite.app, which contains a folder called Logs. A shortcut to this folder is created at Oolite's Start Menu entries during installation. If you have installed the game in a folder subject to file virtualization like C:\Program Files\Oolite and encounter difficulty in locating the log files, then please refer to the Known Issues section of this FAQ for information on how to access them.
Linux and other Unix-like systems: ~/.Oolite/Logs (that is, in the hidden folder .Oolite in your home folder). If you’re using Nautilus or Konqueror—if you’ve never heard of them, you probably are—select Show Hidden Files from the View menu.
Even though the I-Missile carries a smaller than standard warhead, given that missile damage has been re-worked in 1.77 to take relative speeds into account, its' speed of 3.0 LM will mean that significant kinetic damage will be added to the blast damage. This makes it close to being a 'guaranteed-kill' weapon. I'm inclined to think that even the available anti-missile systems would have trouble dealing with it. Think I might do some testing...