It sounds like your manager is at an older version (5.x?) and does not support the newer software version you are looking to load. The manager should be upgraded first.
Once the manager is upgraded the newer versions should show up in the available list of updates. There is also a "manual import" option available under the Manager -> Update Server section, but be sure your manager version supports the version you wish to install by checking the release notes first.
You mention that the M-1250 is at version 5.1.7.x .. which is .x?
Here would be my recommendation based on what I do gather from your email:
1. Upgrade the M-1250 to 22.214.171.124 (can be upgraded from 126.96.36.199 or later)
You should be able to see 188.8.131.52 from the NSM Configure/Manager/Update Server/Software page under the M-1250.
You can just select the 184.108.40.206 version and then click on the Download button to load it to the NSM.
If your manager does not have web access then download that version from the Product Downloads site along with:
M-1250 sensor software 220.127.116.11
Store these files on your local desktop or on the server in any folder you wish... c:\NSP\SSW is good enough.
Then, from the same "Update Server" menu select "Manual Import" and point to each of the files that you downloaded and import them into the manager.
Get select the M-1250 sensors you need to upgrade and push the 18.104.22.168 sensor software to them and reboot them when it tells you to.
Stop .. do not deploy 22.214.171.124 yet.
2. Upgrade the NSM from 126.96.36.199 up to 188.8.131.52
Use the NSM Upgrade guide if you have any questions on the upgrade of the NSM.
General summary is:
a. Perform both an ALL-TABLES and a CONFIG-TABLES backup of the NSM
b. Stop the NSM, NSM UI, and NSM Watchdog services (NSM Database Service should still be running)
c. Run the setup of the 184.108.40.206 NSM software and follow the prompts.
d. If you have more than a million alerts in the database it will tell you to run the offline scripts after the upgrade is completed.
The instructions on running these scripts are better-described in the 7.1 Upgrade guide both in how to run them and what they do
(They take the 220.127.116.11 alert and packetlog schema tables and convert them to the 18.104.22.168 schema)
3. Once the NSM is upgraded to 22.214.171.124 now you can push the 126.96.36.199 Sensor software out to the M-1250.
If you have other sensors that have not yet been upgraded to 188.8.131.52 get them up to that version as well.
This will put you at the prime position to move to 7.1 when you are ready.
The Server hosting the NSM application will HAVE to be at Windows 2008 R2 before you go to NSP Version 7.x
If the Server is a McAfee NSM Appliance (Dell 1950 or R300, or Intel SR1695) currently under support - there is a free upgrade kit to take the server to Windows 2008 R2.