Hello I have written script in POSTMAN but i need to convert it in .jmx file format.if anyone can tell me how i can convert it from JSON to .jmx.
Maybe this might help?
That Loadium page has been broken for a while. We did this using the postmanctl CLI and have an example in that repo.
You can read instructions for downloading postmanctl here: https://github.com/kevinswiber/postmanctl/.
Then you can look at the convert-to-jmeter.sh and jmeter.tmpl files here: https://github.com/kevinswiber/postmanctl/tree/16b378738e3bae6e12056b9e6b7bc94568b44174/examples/go-template.
Essentially, the call just fetches the collection and transforms it through the template file. The command looks something like this.
postmanctl get collection -o go-template-file="./jmeter.tmpl" <collection-uuid>
You can modify the template if needed.
Best,
Kevin
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Loadium is not working for me.
These instructions are not clear for me.
That Loadium page has been broken for a while.
As @kevin.swiber stated 
Please help as Lodium is not working for me .
Hello,
I am from the Loadium team. Sorry for the inconvenience, It might be a network problem or something when you guys tried to use our free converter service at that time. If it happens again please don’t hesitate to contact Loadium support at any time. The Loadium Converter is a free and 7/24 available service, besides it’s an open-source project. You may also freely clone this repository https://github.com/Loadium/postman2jmx and use the converter locally, and please don’t forget to star this repository. 
Just replay it via JMeter’s HTTP(S) Test Script Recorder
If you configure Postman to use JMeter as the proxy and run your test JMeter will capture the requests and convert them into HTTP Request samplers.
More information: How to Convert Your Postman API Tests to JMeter for Scaling
Check this one