You need to be running the application with the meteor run command in one session, at which point you can run mongo meteor in another session on the same machine, which will include something like
[kfullert@shotgun ]$ meteor mongo
MongoDB shell version: 2.2.1
connecting to: 127.0.0.1:3002/meteor
At that point, you can use the URL in the "connecting to" line with the standard mongo tools (caveat - you need to be running your project with meteor at the same time, as "meteor run" is what spins up the mongo server for your project
[kfullert@shotgun ]$ mongo 127.0.0.1:3002/meteor
MongoDB shell version: 2.2.3
connecting to: 127.0.0.1:3002/meteor
>
For mongoimport, you'll probably want something like:
[kfullert@shotgun ]$ mongoimport -h 127.0.0.1 --port 3002 -d meteor
Additionally, it may be possible to run mongoimport without meteor running, by using the following switch from your project root directory (untested so beware)
mongoimport --dbpath .meteor/local/db -d meteor